diff --git a/core/.stylelintrc.json b/core/.stylelintrc.json
index d4d0017b6bb09dfd8cc64d1559ed329dbca0b1ff..e8c83471f45764fa258a58a62ab4d9a10dbe2adb 100644
--- a/core/.stylelintrc.json
+++ b/core/.stylelintrc.json
@@ -4,7 +4,6 @@
     "stylelint-no-browser-hacks/lib"
   ],
   "rules": {
-    "color-hex-case": null,
     "color-hex-length": null,
     "comment-empty-line-before": null,
     "declaration-block-no-duplicate-properties": null,
diff --git a/core/modules/ckeditor/css/ckeditor.css b/core/modules/ckeditor/css/ckeditor.css
index b4b1e16cecc64bee295a330826a25675b0e1559e..bc2c43a6be5be41b84e84b5bee8cafe202de874a 100644
--- a/core/modules/ckeditor/css/ckeditor.css
+++ b/core/modules/ckeditor/css/ckeditor.css
@@ -7,7 +7,7 @@
 
 .ckeditor-dialog-loading-link {
   border-radius: 0 0 5px 5px;
-  border: 1px solid #B6B6B6;
+  border: 1px solid #b6b6b6;
   border-top: none;
   background: white;
   padding: 3px 10px;
diff --git a/core/modules/settings_tray/css/settings_tray.theme.css b/core/modules/settings_tray/css/settings_tray.theme.css
index 7a0e3084eee546d8f3de70ad16e1c5c5e951cacc..d906bda7729d5bbe50ff4d2036abf65576bf6f53 100644
--- a/core/modules/settings_tray/css/settings_tray.theme.css
+++ b/core/modules/settings_tray/css/settings_tray.theme.css
@@ -9,7 +9,7 @@
 
 /* Style the edit mode toolbar and tabs. */
 #toolbar-bar.js-settings-tray-edit-mode {
-  background-image: linear-gradient(to bottom, #0A7BC1, #0a6eb4);
+  background-image: linear-gradient(to bottom, #0a7bc1, #0a6eb4);
 }
 .js-settings-tray-edit-mode .toolbar-item:not(.toolbar-icon-edit) {
   color: #999;
@@ -22,8 +22,8 @@
 .toolbar-tab > .toolbar-icon.toolbar-icon-edit.toolbar-item,
 .toolbar-tab > .toolbar-icon.toolbar-icon-edit.toolbar-item.is-active,
 .toolbar-tab > .toolbar-icon.toolbar-icon-edit.toolbar-item:focus {
-  background-color: #0066A1;
-  background-image: linear-gradient(to bottom, #0066A1, #005b98);
+  background-color: #0066a1;
+  background-image: linear-gradient(to bottom, #0066a1, #005b98);
   color: #eee;
   text-shadow: none;
   font-weight: bold;
@@ -37,8 +37,8 @@
 }
 /* Make the hover of the active state the same as the inactive state. */
 .toolbar-tab > .toolbar-icon.toolbar-icon-edit.toolbar-item.is-active:hover {
-  background-color: #0066A1;
-  background-image: linear-gradient(to bottom, #0066A1, #005b98);
+  background-color: #0066a1;
+  background-image: linear-gradient(to bottom, #0066a1, #005b98);
   color: #fff;
 }
 /* Make the inactive icon grey. */
diff --git a/core/modules/settings_tray/css/settings_tray.toolbar.css b/core/modules/settings_tray/css/settings_tray.toolbar.css
index 7fe5eef3d2d663fdd34cc408215717657f078a4a..4d4880e37a7ba8c992a0b9cf166a1b55208c7c8b 100644
--- a/core/modules/settings_tray/css/settings_tray.toolbar.css
+++ b/core/modules/settings_tray/css/settings_tray.toolbar.css
@@ -20,8 +20,8 @@
 
 /* Style both the edit and editing states of the contextual links toggle tab. */
 .toolbar-icon-edit.toolbar-item {
-  background-color: #0066A1;
-  background-image: linear-gradient(to bottom, #0066A1, #005b98);
+  background-color: #0066a1;
+  background-image: linear-gradient(to bottom, #0066a1, #005b98);
   color: #eee;
   text-shadow: 0 1px hsla(0, 0%, 0%, 0.5);
   font-weight: 700;
diff --git a/core/modules/system/css/system.admin.css b/core/modules/system/css/system.admin.css
index b8f26528f694fcd699bd3ef7fd4f9976db04f84e..a6b9e568e91b2737c8ad1cab69bc43cbb8c98a2b 100644
--- a/core/modules/system/css/system.admin.css
+++ b/core/modules/system/css/system.admin.css
@@ -117,7 +117,7 @@ small .admin-link:after {
   text-transform: none;
 }
 .system-modules td details a {
-  color: #5C5C5B;
+  color: #5c5c5b;
   border: 0;
 }
 .system-modules td details {
diff --git a/core/modules/views_ui/css/views_ui.admin.theme.css b/core/modules/views_ui/css/views_ui.admin.theme.css
index e1f4d7a405eb01ae7f8294b66dfd7bda8cf149a4..ed21b8981616232240f00a86182092934e08a974 100644
--- a/core/modules/views_ui/css/views_ui.admin.theme.css
+++ b/core/modules/views_ui/css/views_ui.admin.theme.css
@@ -624,10 +624,10 @@ td.group-title {
   border: none;
 }
 .views-ui-dialog .views-offset-top {
-  border-bottom: 1px solid #CCC;
+  border-bottom: 1px solid #ccc;
 }
 .views-ui-dialog .views-offset-bottom {
-  border-top: 1px solid #CCC;
+  border-top: 1px solid #ccc;
 }
 .views-ui-dialog .views-override > * {
   margin: 0;
diff --git a/core/profiles/demo_umami/themes/umami/css/base.css b/core/profiles/demo_umami/themes/umami/css/base.css
index a8ef6a2fa75ac55e4aa7a72b6dcba2de7b7eff53..0a25962cfff43ed005365507103b3abc48a76542 100644
--- a/core/profiles/demo_umami/themes/umami/css/base.css
+++ b/core/profiles/demo_umami/themes/umami/css/base.css
@@ -14,7 +14,7 @@ html {
 }
 
 a {
-  color: #00836D;
+  color: #00836d;
   text-decoration: underline;
 }
 a:hover,
diff --git a/core/profiles/demo_umami/themes/umami/css/components/blocks/banner/banner.css b/core/profiles/demo_umami/themes/umami/css/components/blocks/banner/banner.css
index 0418fbcbf4fccc9a5c359afc2973bbe2ce789df1..34dcb16d3a5716a5ec4a53b91af414b4a66ea463 100644
--- a/core/profiles/demo_umami/themes/umami/css/components/blocks/banner/banner.css
+++ b/core/profiles/demo_umami/themes/umami/css/components/blocks/banner/banner.css
@@ -27,7 +27,7 @@
   padding: 0.5em 1.5em 0.4em;
   border: 0;
   border-radius: 4px;
-  background-color: #E84265;
+  background-color: #e84265;
   color: #fff;
   cursor: pointer;
   text-align: center;
@@ -41,7 +41,7 @@
 
 .block-banner-block .field--name-field-content-link a:focus,
 .block-banner-block .field--name-field-content-link a:hover {
-  background-color: #00836D;
+  background-color: #00836d;
   color: #fff;
   text-decoration: underline;
 }
diff --git a/core/profiles/demo_umami/themes/umami/css/components/blocks/quicklinks/quicklinks.css b/core/profiles/demo_umami/themes/umami/css/components/blocks/quicklinks/quicklinks.css
index d9fe06b84fa4a97c3a22eb0d8b7aa1cbba8eb779..3986c47c33d3c3113448fcbc8609e68aa1e429d3 100644
--- a/core/profiles/demo_umami/themes/umami/css/components/blocks/quicklinks/quicklinks.css
+++ b/core/profiles/demo_umami/themes/umami/css/components/blocks/quicklinks/quicklinks.css
@@ -50,7 +50,7 @@
   height: 1px;
   width: 80%;
   content: '';
-  background-color: #E6EEE0;
+  background-color: #e6eee0;
 }
 .quicklinks__col:last-child::after {
   display: none;
@@ -71,7 +71,7 @@
     height: 100%;
     width: 1px;
     content: '';
-    background-color: #E6EEE0;
+    background-color: #e6eee0;
   }
   .quicklinks__col:nth-child(3)::after {
     display: block;
diff --git a/core/profiles/demo_umami/themes/umami/css/components/blocks/search/search.css b/core/profiles/demo_umami/themes/umami/css/components/blocks/search/search.css
index 360f629e33639fe76bb4342dbee0d60178a916c2..8df9819aca6ff91387ccc04f1f68899e60f72ebd 100644
--- a/core/profiles/demo_umami/themes/umami/css/components/blocks/search/search.css
+++ b/core/profiles/demo_umami/themes/umami/css/components/blocks/search/search.css
@@ -16,7 +16,7 @@
     flex: 0 1 50%;
     order: 1;
     display: block;
-    border-bottom: 1px solid #E6EEE0;
+    border-bottom: 1px solid #e6eee0;
   }
 }
 
diff --git a/core/profiles/demo_umami/themes/umami/css/components/content-types/recipe/recipe.css b/core/profiles/demo_umami/themes/umami/css/components/content-types/recipe/recipe.css
index 43de426cbb1299b81c37d78afec71910670ad7e1..fc88757bec583e82aff185a9b316c91f2ef46902 100644
--- a/core/profiles/demo_umami/themes/umami/css/components/content-types/recipe/recipe.css
+++ b/core/profiles/demo_umami/themes/umami/css/components/content-types/recipe/recipe.css
@@ -190,7 +190,7 @@
   line-height: 1.2;
   padding: 0 0 0.6em 0;
   margin: 0 0 1em 0;
-  border-bottom: 1px solid #EEC2CB;
+  border-bottom: 1px solid #eec2cb;
 }
 /* Large */
 @media screen and (min-width: 60rem) { /* 960px */
diff --git a/core/profiles/demo_umami/themes/umami/css/components/content/full/node-full.css b/core/profiles/demo_umami/themes/umami/css/components/content/full/node-full.css
index 5d8c6cf572d17b964232b4115e02f3a9cd93beef..e431e444eadcb3f0c13b564bf70a97f841d657c0 100644
--- a/core/profiles/demo_umami/themes/umami/css/components/content/full/node-full.css
+++ b/core/profiles/demo_umami/themes/umami/css/components/content/full/node-full.css
@@ -47,7 +47,7 @@
   vertical-align: middle;
   display: inline-block;
   margin: 0 0.25ex;
-  color: #7CBE8C;
+  color: #7cbe8c;
 }
 
 .node--view-mode-full .field--name-field-tags {
diff --git a/core/profiles/demo_umami/themes/umami/css/components/navigation/menu-account/menu-account.css b/core/profiles/demo_umami/themes/umami/css/components/navigation/menu-account/menu-account.css
index eadd7dc114716d417baa41328797bb7bd7a7f1d9..04f850828580fb98cc7b6a59e57a6d57656ed2bc 100644
--- a/core/profiles/demo_umami/themes/umami/css/components/navigation/menu-account/menu-account.css
+++ b/core/profiles/demo_umami/themes/umami/css/components/navigation/menu-account/menu-account.css
@@ -12,7 +12,7 @@
     flex: 0 1 50%;
     order: 2;
     display: block;
-    border-bottom: 1px solid #E6EEE0;
+    border-bottom: 1px solid #e6eee0;
     text-align: right;
   }
 }
diff --git a/core/themes/bartik/css/components/captions.css b/core/themes/bartik/css/components/captions.css
index cd248094f72cd49b93db90e6f03118b1a7cef36d..1564e8be02248450562dc5397198e78b096ec746 100644
--- a/core/themes/bartik/css/components/captions.css
+++ b/core/themes/bartik/css/components/captions.css
@@ -3,12 +3,12 @@
   margin-bottom: 1.2em;
 }
 .caption > * {
-  background: #F3F3F3;
+  background: #f3f3f3;
   padding: 0.5ex;
-  border: 1px solid #CCC;
+  border: 1px solid #ccc;
 }
 .caption > figcaption {
-  border: 1px solid #CCC;
+  border: 1px solid #ccc;
   border-top: none;
   padding-top: 0.5ex;
   font-size: small;
diff --git a/core/themes/bartik/css/components/views.css b/core/themes/bartik/css/components/views.css
index 61712577e8b4589af8d431d812ce7c20d551e067..1e45f5795399892c987a6d26d552ff1378e8fa99 100644
--- a/core/themes/bartik/css/components/views.css
+++ b/core/themes/bartik/css/components/views.css
@@ -9,7 +9,7 @@
 }
 .views-displays .tabs .open > a:hover,
 .views-displays .tabs .open > a:focus {
-  color: #0071B3;
+  color: #0071b3;
 }
 .views-displays .secondary .form-submit {
   font-size: 0.846em;
@@ -22,14 +22,14 @@
 
 /* Contextual filter options styles */
 .views-filterable-options .filterable-option:nth-of-type(even) .form-type-checkbox {
-  background-color: #F9F9F9;
+  background-color: #f9f9f9;
 }
 
 /* Views action dropbutton styles */
 .views-ui-display-tab-actions .dropbutton .form-submit {
-  color: #0071B3;
+  color: #0071b3;
 }
 .views-ui-display-tab-actions .dropbutton .form-submit:hover,
 .views-ui-display-tab-actions .dropbutton .form-submit:focus {
-  color: #018FE2;
+  color: #018fe2;
 }
diff --git a/core/themes/seven/css/components/buttons.css b/core/themes/seven/css/components/buttons.css
index 6d0d50fbc94f5f8974931b8267c0ece0c1aff59e..79b0fe4aa40d9037e3625e4a001e07e7b8097012 100644
--- a/core/themes/seven/css/components/buttons.css
+++ b/core/themes/seven/css/components/buttons.css
@@ -62,7 +62,7 @@
 /* Prevent focus ring being covered by next siblings. */
 .button:focus {
   z-index: 10;
-  border: 1px solid #3AB2FF;
+  border: 1px solid #3ab2ff;
   box-shadow: 0 0 0.5em 0.1em hsla(203, 100%, 60%, 0.7);
 }
 .button:active {
@@ -94,7 +94,7 @@
   color: #fff;
 }
 .button--primary:focus {
-  border: 1px solid #1280DF;
+  border: 1px solid #1280df;
 }
 .button--primary:hover {
   box-shadow: 0 1px 2px hsla(203, 10%, 10%, 0.25);
diff --git a/core/themes/seven/css/components/form.css b/core/themes/seven/css/components/form.css
index b7cb3b8f9cf5cc10c5b62aef4594e9fb7e2d7c34..9f27d310c97eac456c74c27d9e539353a279f8b8 100644
--- a/core/themes/seven/css/components/form.css
+++ b/core/themes/seven/css/components/form.css
@@ -241,6 +241,7 @@ select {
     background:
       url(../../../../misc/icons/333333/caret-down.svg) no-repeat 99% 63%,
       -webkit-linear-gradient(top, #f6f6f3, #e7e7df); /* LTR */
+
     text-shadow: 0 1px hsla(0, 0%, 100%, 0.6);
     font-size: 0.875rem;
     -webkit-transition: all 0.1s;
diff --git a/core/themes/seven/css/components/jquery.ui/theme.css b/core/themes/seven/css/components/jquery.ui/theme.css
index 97390910074b9162c11cc8214c82fa0a6d6482b3..4c057571f15fd29ac88edcb7d3124cad5fce2bc4 100644
--- a/core/themes/seven/css/components/jquery.ui/theme.css
+++ b/core/themes/seven/css/components/jquery.ui/theme.css
@@ -347,8 +347,8 @@
 .ui-slider .ui-slider-handle {
   border: 1px solid #e4e4e4;
   border-bottom: 1px solid #b4b4b4;
-  border-left-color: #D2D2D2;
-  border-right-color: #D2D2D2;
+  border-left-color: #d2d2d2;
+  border-right-color: #d2d2d2;
   background-color: #e4e4e4;
   border-radius: 4px;
 }
@@ -375,16 +375,16 @@
  * Date Picker
  */
 .ui-datepicker {
-  border: 1px solid #A6A6A6;
-  background: #FFF;
+  border: 1px solid #a6a6a6;
+  background: #fff;
   /* Override datepicker.css */
   padding: 0;
 }
 
 /* Override tables.css */
 .ui-datepicker-calendar thead tr {
-  border-bottom: 1px solid #A6A6A6;
-  border-top: 1px solid #A6A6A6;
+  border-bottom: 1px solid #a6a6a6;
+  border-top: 1px solid #a6a6a6;
 }
 .ui-datepicker-calendar tr:hover {
   background: transparent;
diff --git a/core/themes/seven/css/components/system-status-counter.css b/core/themes/seven/css/components/system-status-counter.css
index 5205c912a27f26e8dbe33e60eb89d00779298297..86f9668927aeac438a350eb3a3e037703d1b72e1 100644
--- a/core/themes/seven/css/components/system-status-counter.css
+++ b/core/themes/seven/css/components/system-status-counter.css
@@ -11,7 +11,7 @@
   display: inline-block;
   width: 100%;
   white-space: nowrap;
-  background: #FCFCFA;
+  background: #fcfcfa;
 }
 .system-status-counter__status-icon {
   display: inline-block;
diff --git a/core/themes/seven/css/components/tabs.css b/core/themes/seven/css/components/tabs.css
index e9aa26a926350c30883111cf047ae7fa2f3ec52e..424f6382d0ac16ee0319c52e6f59b04715ac350f 100644
--- a/core/themes/seven/css/components/tabs.css
+++ b/core/themes/seven/css/components/tabs.css
@@ -9,7 +9,7 @@
 .is-horizontal .tabs:before {
   content: '';
   display: block;
-  background-color: #A6A6A6;
+  background-color: #a6a6a6;
   height: 1px;
   position: absolute;
   bottom: 0;
diff --git a/core/themes/seven/css/components/views-ui.css b/core/themes/seven/css/components/views-ui.css
index d212dd0bf9ad4f18f88eae9c85cb01a9ddcba07f..83273b96366061b112db9ce94eddfb4a1c2f27cb 100644
--- a/core/themes/seven/css/components/views-ui.css
+++ b/core/themes/seven/css/components/views-ui.css
@@ -157,7 +157,7 @@ details.fieldset-no-legend {
 /* @group Attachment details */
 
 #edit-display-settings-title {
-  color: #008BCB;
+  color: #008bcb;
 }
 
 /* @end */
@@ -189,7 +189,7 @@ details.fieldset-no-legend {
 .views-displays .secondary .open > a:hover,
 .views-displays .secondary .open > a:focus {
   background-color: #f1f1f1;
-  color: #008BCB;
+  color: #008bcb;
 }
 
 .views-displays .secondary .action-list li:first-child {
@@ -266,11 +266,11 @@ details.fieldset-no-legend {
 }
 
 .views-ui-rearrange-filter-form tr.drag td {
-  background-color: #FFEE77 !important;
+  background-color: #ffee77 !important;
 }
 
 .views-ui-rearrange-filter-form tr.drag-previous td {
-  background-color: #FFFFBB !important;
+  background-color: #ffffbb !important;
 }
 
 /* @end */
diff --git a/core/themes/seven/css/theme/ckeditor-dialog.css b/core/themes/seven/css/theme/ckeditor-dialog.css
index a7870d87aa876141e21180a984e3a72c59db623c..e791a2e155aaffd25d7eb28c555b1cddd1ff0193 100644
--- a/core/themes/seven/css/theme/ckeditor-dialog.css
+++ b/core/themes/seven/css/theme/ckeditor-dialog.css
@@ -196,7 +196,7 @@
 }
 .cke_reset_all .cke_dialog_footer_buttons a.cke_dialog_ui_button:focus {
   z-index: 10;
-  border: 1px solid #3AB2FF;
+  border: 1px solid #3ab2ff;
   box-shadow: 0 0 0.5em 0.1em hsla(203, 100%, 60%, 0.7);
 }
 .cke_reset_all .cke_dialog_footer_buttons a.cke_dialog_ui_button:active {
diff --git a/core/themes/stable/css/ckeditor/ckeditor.css b/core/themes/stable/css/ckeditor/ckeditor.css
index b4b1e16cecc64bee295a330826a25675b0e1559e..bc2c43a6be5be41b84e84b5bee8cafe202de874a 100644
--- a/core/themes/stable/css/ckeditor/ckeditor.css
+++ b/core/themes/stable/css/ckeditor/ckeditor.css
@@ -7,7 +7,7 @@
 
 .ckeditor-dialog-loading-link {
   border-radius: 0 0 5px 5px;
-  border: 1px solid #B6B6B6;
+  border: 1px solid #b6b6b6;
   border-top: none;
   background: white;
   padding: 3px 10px;
diff --git a/core/themes/stable/css/system/system.admin.css b/core/themes/stable/css/system/system.admin.css
index e8f8f5993ea93592b708f835f725efdd291ecf5a..24b41dc20eaddb37cd5b7961938c6baa8a25b984 100644
--- a/core/themes/stable/css/system/system.admin.css
+++ b/core/themes/stable/css/system/system.admin.css
@@ -117,7 +117,7 @@ small .admin-link:after {
   text-transform: none;
 }
 .system-modules td details a {
-  color: #5C5C5B;
+  color: #5c5c5b;
   border: 0;
 }
 .system-modules td details {
diff --git a/core/themes/stable/css/views_ui/views_ui.admin.theme.css b/core/themes/stable/css/views_ui/views_ui.admin.theme.css
index d405ba63da8cee3845982856b239decfeb23973f..7ceec0aec37f33b52c132dcf6e4c786827268651 100644
--- a/core/themes/stable/css/views_ui/views_ui.admin.theme.css
+++ b/core/themes/stable/css/views_ui/views_ui.admin.theme.css
@@ -624,10 +624,10 @@ td.group-title {
   border: none;
 }
 .views-ui-dialog .views-offset-top {
-  border-bottom: 1px solid #CCC;
+  border-bottom: 1px solid #ccc;
 }
 .views-ui-dialog .views-offset-bottom {
-  border-top: 1px solid #CCC;
+  border-top: 1px solid #ccc;
 }
 .views-ui-dialog .views-override > * {
   margin: 0;