CSS tweaks/fixes/night mode

Night mode styling/colours is open to change as ever. Minor changes for the most part though some pages/states of reddit may have been missed. Mainly removal of !important in favour of (sometimes) more specific selectors.

General CSS clean up, nothing major, mainly small stuff. 0 instead of 0px, merging multiples into shorthand, removal of moz/webkit prefixes for border-radius since they're obsolete.

Fix for the notifications being partially overlapped by the header.