This reverts commit b7e0d9b1.

more in-depth documentation of slide backgrounds #1542

make theme "simple" use white titles on dark backgrounds

set/remove @disabled on navigation buttons

make speaker notes keyboard accessible

Add selection colour for Mozilla Firefox

Found that by running css-lint on the code

Places div.speaker-notes in default kbd Tab order, and when focused,
prevent slide navigation with up/down arrows, allowing up/down arrow
keys to scroll div.speaker-notes.

Keeps disabled buttons out of kbd tab order and indicates to assistive
tech like screen readers that button is disabled. Otherwise buttons
that aren’t enabled remain in kbd Tab order and screen readers announce
them as buttons, but they don’t work.

Simplify full setup

More natural zooming on block level elements