Dark theme fixes 2023 (#131)

* Fix image captions being invisible when inheriting light theme color

* Make selection not white on white in CodeMirror

* Make File: metadata not white on white

* Fix various white-on-white issues in VisualEditor

* Fix various white-on-white issues in page history

* Fix pending-stripes visual interference

* Generalize some OOUI fixes

* Fix CodeMirror gutter being white

* Fix search results being white

* Make WikiEditor help table not dark on dark

* Make CodeMirror extlinks not dark on dark

* Fix preview warning being too bright

* Don't add borders to every pseudo-input

* Don't give scratchblocks a background

* Fix general message box styling

* Fix WikiEditor special characters menu dark on dark

* Fix page history selected item dark on dark

* Fix many more VisualEditor/OOUI issues

* Generally more consistent OOUI border colors

* Fix Special:Preferences

* A little rearrangement

* Recent Changes fixes

* Fix VisualEditor popups having zero height

* Fix various non-dark-Echo issues

* Fix white-on-white reference highlighting