Reapply void-docs theme changes

The contents of this commit should closely follow the patch linked
in the "Void's changes to mdBook's default theme" section of
#840

Some things had to be adapted to the newer v0.4.43 base. Some changes
include:

- The warning and kbd elements were left intact in general.css
- -webkit-print-color-adjust was set in the pre element to match
  void-docs' old theme even though the new theme's code element no
  longer includes it in print.css
- Default code fonts were left as-is (these weren't present here in the
  void-docs original theme) in variables.css
- Warning border was left as-is (this element wasn't present in older
  versions of mdBook) in variables.css
- copy-button-filter variables were left as-is in variables.css
- variables.css no doesn't include special prefers-color-scheme: dark
  noscript logic (void-docs original theme didn't have it)

Author: meator