Continued from T91152.
OOUI uses font-size of 12.8px (0.8×16px) for windows and toolbars, but Vector skin uses 14px (0.875×16px) for body content, creating inconsistency between widgets in windows/toolbars and "free-standing" ones.
We should change OOUI MediaWiki theme to be optimized for font-size of 14px (0.875em×16px), instead of 12.8px (0.8em×16px), and make sure that this doesn't explode existing interfaces. To avoid redrawing all existing icons with new grid, we'll keep the current absolute size for them (making them slightly smaller relative to font size).
Proposal (also featuring overhauled icons from T177432):