Issue #3112452 by lalitware, guilhermevp, narendra.rajwar27, WagnerMelo, ravi.shankar, mradcliffe, adalbertov, himanshu_sindhwani, quondam, Vidushi Mehta, alexpott, longwave, anmolgoyal74: Fix indentation consistency in core's yaml files

Issue #3097890 by martin107, longwave, andypost, lauriii: Remove all @deprecated code in core themes

Issue #3072702 by alexpott, Wim Leers, mikelutz, xjm, catch, Berdir, tedbow, webchick, shaal, Mixologic, heddn: Core extensions should not need to specify the new core_version_requirement in *.info.yml files so that 9.0.x can be installed

Issue #2866817 by harsha012, Jo Fitzgerald, BrightBold, alexpott, joelpittet: Update stylelint rule no-empty-source to be consistent with Drupal's CSS standards

Issue #2845731 by ashish-deynap, vaplas, chiranjeeb2410, Munavijayalakshmi, tramsaal, xjm: Theming guide links are wrong

Issue #2575421 by Cottser, mdrummond, lauriii, davidhernandez, LewisNyman, catch, alexpott, joelpittet, webchick, Bojhan, emma.maria, mortendk: Add a Stable base theme to core and make it the default if a base theme is not specified

Issue #2424805 by mdrummond, YesCT, vijaycs85, xjm, marcoscano, ifrik, joelpittet, Jelle_S, attiks, lauriii: Toolbar can no longer switch horizontal and vertical -- expects breakpoints ordered from smallest to largest; responsive images need largest to smallest

Issue #2349711 by lauriii, mortendk, sqndr, akalata, yannickoo, Manjit.Singh, Jolidog, b0unty, aliyakhan, LewisNyman: Remove all visual from stark

Issue #2491155 by mikeburrelljr, Mac_Weber, opdavies: Update drupal.org and kernel.org URLs in core modules (Follow-up to 2489912)

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 #2459971 by Gábor Hojtsy: The langcode key on configuration files is not explicit in configuration schema

Issue #2260061 by Jelle_S, attiks, Wim Leers, mdrummond, mgifford, holist: Responsive image module does not support sizes/picture polyfill 2.2

Issue #2142653 by corbacho, sqndr, LewisNyman, floretan, BarisW, G-raph, JamesLefrère, lokapujya, LoMo, Outi: Change default logo filetype to .svg and add an SVG version of Druplicon

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

Issue #2271529 by attiks, alexpott, Lowell, YesCT, Jelle_S | mdrummond: Move breakpoint settings to theme and module *.breakpoints.yml files at root level.

Issue #1912610 by amitgoyal, BryanCGreen24, lokapujya, Rajendar Reddy, echoz, barraponto, LewisNyman, pixelwhip: Rename layout classes to follow the layout style naming convention.

Issue #2233787 by alexpott, xjm | tim.plunkett: Move default configuration into extension/config/install to clarify its purpose.

Issue #2218655 by sun, effulgentsia, jessebeach: Core expects $theme.settings default config file to exist.

Issue #922696 by mgifford, EvanDonovan, LewisNyman, stpaultim, jensimmons: New methods, standards for describing core themes on themes admin page (was Theme screenshots do not have adequate alt text...).

Issue #2167623 by danilenko_dn, sidharthap, Nitesh Sethia, krishnan.n, aitiba, alexpott, ashwinikumar, Barrett, damiankloip, deepakaryan1988, foxtrotcharlie, ianthomas_uk, neetu morwani, nonsie, piyuesh23, Sharique, sivaji, sushantpaste, swentel, vijaycs85, YesCT: Add test for all default configuration to ensure schema exists and is correct.

Issue #2117441 by alanburke: Fixed Stark layout broken as 'generic' classes removed from html.tpl.php.

Issue #2011578 by mortendk, tlattimore, Cottser, soulston: Markup for Stark's page.html.twig + maintenance-page.html.twig.

Issue #2015789 by fubhy, echoz, rteijeiro, LewisNyman, penyaskito: Remove language_css_alter() (RTL stylesheets) in favor of HTML 'dir' attribute.

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

Issue #1276908 by jessebeach, nod_, moshe weitzman, lewisnyman, webchick: Fixed Administrative tables are too wide for smaller screens.

- Patch #1645494 by effulgentsia: Allow core themes to display single-column on IE8: do not compensate for lack of media query support.

- Patch #1322794 by scor: follow up patch fixing the 'No newline at end of file' in ie-rtl.css and ie.css.