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 #2922701 by quietone, masipila, heddn, benjifisher, phenaproxima, larowlan, xjm, catch: Migrate UI - refer to modules and add help text

Issue #2905227 by maxocub, yoroy, jjpoole, heddn, phenaproxima, vulcanr, quietone, wturrell, xjm: Migrate UI: Improve 'Review Upgrade' page UX