Issue #3343212 by markconroy, andy-blum, bnjmnm, longwave, alexpott, lauriii, codexmas, harsha012, rpayanm, occupant, Mukeysh, leahtard, joelpittet: Update Core CSS to use double-colon for pseudo elements

Issue #2209063 by bnjmnm, Sam152, tarekdj, lauriii, Wim Leers, LewisNyman, quietone: Fix the flicker of the "active" icons when a high-latency user clicks on toolbar items

Issue #2866801 by harsha012, joelpittet, BrightBold, droplet, Akshay kashyap, arunkumark, bmx269: Update stylelint rules color-hex-length to be consistent with Drupal's CSS standards

Issue #2485375 by marieke_h, Manjit.Singh, MathieuSpil, LewisNyman, geertvd: Clean up shortcut CSS inline with our CSS standards

Issue #2031641 by aburrows, lauriii, nlisgo, Pol, redsquid, tuutti, LewisNyman, akalata, agviu, kallehauge, _nolocation, rpayanm, saki007ster, mdrummond, RavindraSingh, brahmjeet789: Change active class to is-active

Issue #2309137 by longwave, sqndr | LewisNyman: Remove quotes from all background-image: url() properties.

Issue #1490402 by LewisNyman, nod_, mcjim, mdrummond, rteijeiro, jessebeach, Bojhan, ry5n: Redesign tabs and the content header.

Issue #1963886 by ry5n, jessebeach, dcrocks, Wim Leers: Use HiDPI icons in the toolbar to support scaling on different devices.

Issue #1938044 by hass, jessebeach, Dragan Eror: Fixed Prefix all toolbar classes to prevent theme clashes.

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

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