UX: Remove box-shadow from kbd, unify styling (PR #9948)

Having many elements with multiple box-shadows (e.g. in keyboard shortcuts modal) was tanking scrolling performance in some browsers.

Before/after

before_light_help after_light_help
before_light_post after_light_post
before_dark_help after_dark_help
before_dark_post after_dark_post

GitHub

LGTM, we should merge this before beta