Issue #3073053 by pivica, Primsi, unstatu, pooja saraah, smustgrave, Berdir: Theme library override can fail in when you have multiple parent themes

Issue #3069052 by mikelutz, Berdir, longwave, ravi.shankar, voleger, lauriii, catch, andypost: Properly deprecate the stylesheets-remove key from theme info.yml files

Issue #3113053 by hash6, Hardik_Patel_12, longwave, alexpott: Remove $baseThemes BC layer from \Drupal\Core\Theme\ActiveTheme

Issue #3048498 by jonathan1055, Lendude, naveenvalecha, klausi, alexpott, Gábor Hojtsy: [≈Nov. 11] Fix Drupal.Commenting.Deprecated coding standard

Issue #3049099 by alexpott, catch: Whilst updating from 8.6.x to 8.7.x you get many "Invalid argument supplied for foreach() in Drupal\Core\Theme\ThemeInitialization" warnings

Issue #2659940 by almaudoh, alexpott, phenaproxima, Jo Fitzgerald, markcarver, oriol_e9g, dawehner, dsnopek, jibran, larowlan: Extension System, Part III: ThemeExtensionList and ThemeEngineExtensionList

Issue #1507896 by b0unty, tuutti, Jeff Burnz, LewisNyman, tameeshb, realityloop, cbanman, umarzaffer, manumilou, mgifford, mducharme, oakulm, lauriii, alexpott, joelpittet, MaskyS, markcarver, xjm: Allow theme developers to add the default logo filename to the theme's .info.yml

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2497667 by almaudoh, Wim Leers, Cottser, Fabianx, jaxxed: Add libraries-extend to themes' *.info.yml

Issue #2451411 by almaudoh, Shamsher_Alam, lauriii, borisson_, cilefen, davidhernandez, Cottser, Wim Leers, joelpittet: Add libraries-override to themes' *.info.yml

Issue #2497259 by catch, tim.plunkett, alexpott: system_region_list() unnecessarily translates region names

Issue #2389735 by lauriii, iMiksu, davidhernandez, larowlan, loopduplicate, LewisNyman, Wim Leers, dawehner, Cottser: Core and base theme CSS files in libraries override theme CSS files with the same name

Issue #2414255 by lauriii, Jeff Burnz, dawehner, davidhernandez: Subtheme template inheritance working in reverse order

git commit -m 'Issue #2377397 by Wim Leers, alexpott: Themes should use libraries, not individual stylesheets'