Issue #3374901 by e0ipso, mglaman, smustgrave, mherchel, _utsavsharma,...

Issue #3374901 by e0ipso, mglaman, smustgrave, mherchel, _utsavsharma, gareth.poole, catch, lauriii, eelkeblok, web-beest: SDC stylesheets should be added in the "theme" aggregate group (as opposed to "default" group) to correct CSS source order

(cherry picked from commit c98044ce)