Issue #2013094 by pwieck, Cottser: Remove unnecessary template_preprocess() reference from all Twig files

Issue #2003814 by dasjo, pmelab, Cottser: Remove Bartik process layer functions, refactor color module alterations.

Issue #2018913 by ekl1773, jessebeach: Vertical tab styling for tabs should not be applied when JavaScript is disabled and no tabs are present.

Issue #1363112 by mparker17, mgifford, tlattimore, Jacine, dcmouyard, bowersox, ry5n, ahdiaz: Simplify names of 'element-x' helper classes.

Issue #1861702 by echoz, rteijeiro, LewisNyman, mcjim: Add a responsive grid to the Appearance page.

Issue #1987066 by Kevin Morse, JohnAlbin, rteijeiro, echoz, ry5n: Rename files to match CSS file naming convention.

Issue #1932948 by falcon03: Primary tabs could not work in certain situations due to a wrong ARIA role.

Issue #1806478 by Cottser, joelpittet, kevinquillen: Make twig the default engine once all modules templates are converted from .tpl.php to .html.twig.

Issue #1938840 by chrisjlee, duellj, Cottser, shanethehat, idflood, tlattimore: Bartik.theme - Convert PHPTemplate templates to Twig.

Issue #1938848 by Jaesin, Cottser, jenlampton, hefox, Shawn DeArmond, frob, boze, joelpittet, mrf, dcrocks: seven.theme - Convert PHPTemplate templates to Twig.

Issue #1961870 by Cottser, foopang, kgoel, shanethehat, tlattimore, c4rl, joelpittet, thedavidmeister: Convert page.tpl.php to Twig.

Issue #1712250 by alexpott, n3or, cam8001, vijaycs85, Albert Volkman, jibran, heyrocker, chx: Convert theme settings to configuration system.

Issue #1913618 by tim.plunkett, amateescu: Convert EntityFormControllerInterface to extend FormInterface.

Issue #1975462 by thedavidmeister, alexpott, chx, joelpittet: Fixed Allow and test for NULL and integer 0 values in Twig templates.

Issue #1973456 by echoz: Fixed .element-invisible on element at right causes horizontal scrollbar on desktop or right gap on phone on webkit browsers.

Issue #1937824 follow-up by echoz, mariacha1: Simpler fix for Improve button-link spacing on confirm form.

Issue #1033116 by eojthebrave, c4rl, brianV, tim.plunkett | webchick: Rename template.php to THEMENAME.theme to elimnate ambiguity around 'the template file' vs. 'template files'.

Issue #1940208 by pameeela, mgifford, echoz: Fixed Contrast for add & remove shortcut links on overlay not sufficient for WCAG - Regression.

Issue #1937824 by mariacha1, keyhitman, pixelmord, pameeela: Fixed Improve button-link spacing on confirm form.

Issue #1924368 by JohnAlbin, ykhadilkar, carwin: Change notice: Rename old CSS constants to CSS_AGGREGATE_* and add new CSS weight constants.

Issue #1847084 by jessebeach, trawekp, LewisNyman, nod_, tarekdj: Fixed Vertical toolbar doesn't work with overlay (measure/track displacing elements better + provide change events for them).

Issue #1913958 by hass, YesCT: Fixed Bartik theme: Don't change capitalization of translatable strings with CSS.