Issue #2485425 by MathieuSpil, iro, saki007ster, Wim Leers, Manjit.Singh, wiifm, kostask, Studiographene, alvar0hurtad0, LewisNyman: Clean up editor CSS inline with our CSS standards

Issue #2566771 by Cottser, mortendk, jstoller, LewisNyman, mdrummond, Gints Ērglis, sqndr, Chernous_dn, Manjit.Singh, axe312, emma.maria, davidhernandez, lauriii, jaxxed, akalata: [Voltron patch] Move all remaining *.theme.css to Classy

Issue #2566597 by Cottser, LewisNyman, mdrummond, pjbaert, jaxxed, ManuelGarcia, emma.maria, maked1sky, davidhernandez: [Mega patch] Move system *.theme.css files to Classy

This reverts commit 7cabfeee.

This reverts commit fb07e7bb.

Issue #2553465 by mdrummond, pguillard, LewisNyman: Move system exposed-filters.theme.css, more-link.theme.css, and pager.theme.css to Classy

Issue #2489578 by Manjit.Singh, saki007ster, pjbaert, Cottser, jhodgdon, epophoto: Move search.theme.css to classy

Issue #2228745 by Wim Leers, sqndr, nod_, undertext: Fixed Color module doesn't look for CSS files declared in a library.