Prevent innacurate "insufficent contrast" errors from scanners looking at visually hidden elements

Problem/Motivation

I ran into this with the Wave scanner. While there's no user need to have sufficient contrast on visually hidden elements, if a scanner reports a contrast error it gives the impression that Gin is less accessible than it really is. While it's true this is ultimately a problem with the A11y scanner, we can update the text color of visually hidden elements pretty easily and in a non-disruptive way, so why not do this to ensure Gin accessibility scans do not report irrelevant errors.

Steps to reproduce

Run the Wave scanner on any page with the secondary toolbar present and note the contrast errors https://wave.webaim.org/extension/

Run that same scanner on pretty much any page in dark mode and note even more contrast errors.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes