Change class affecting ::backdrop style

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

PASS getComputedStyle(dialog, '::backdrop').backgroundColor is not green
PASS internals.updateStyleAndReturnAffectedElementCount() is 2
PASS getComputedStyle(dialog, '::backdrop').backgroundColor is green
PASS successfullyParsed is true

TEST COMPLETE