For that:
* Tweak the standin system colors to match the non-native theme.
* Use those system colors for button and field backgrounds.
* Rename the "should use system colors" bit to "is high contrast",
which is what it really is (specially now that we use system colors
also in non-high-contrast).
Border colors and other colors like the <input type=range> and such
might need some extra tweaking perhaps, but this is a decent start and
looks good in https://crisal.io/tmp/form-controls.html afaict (dark mode
toggle needs the color-scheme pref enabled of course).
Differential Revision: https://phabricator.services.mozilla.com/D127533
