Background
In T363849 we discovered that despite the hard work undertaken in integrating Codex 1.5 into the OOUI Wikimedia theme, that OOUI icons still need to be adapted to support dark-mode.
User story
As someone using OOUI products like VisualEditor, I expect icons to be visible in dark-mode.
Current experience (based on in-progress work from T363849 and seen on patchdemo: https://patchdemo.wmflabs.org/wikis/717f060a19/w/index.php?title=Main_Page&veaction=edit
Requirements
Icons used in OOUI should be visible in dark-mode.
Design
I propose that OOUI icons support dark-mode via the .skin-invert class, which will have the impact of giving OOUI icons a slightly different (incorrect) color in dark-mode, but at least it will make icons that are black in light-mode, white in dark-mode.
Proposed solution:
