https://searchfox.org/mozilla-central/source/browser/themes/shared/customizableui/customizeMode.inc.css has a handful of per-platform ifdefs. Now that bug 1754547 landed, it should be straightforward to replace them with media queries and stop pre-processing it.

It looks like we then include it on a per-platform basis in browser.css. Instead, we could have it be a standalone stylesheet and load it dynamically when entering customize mode for the first time (and await the inserted stylesheet loading, so we don't hit a flash of unstyled content), from CustomizeMode.jsm. At least at a glance, I don't think any of the rules in that sheet apply to anything outside of customize mode.

Replaced OS specific preprocessor conditionals in customizeMode.inc.css with media queries.

https://hg.mozilla.org/mozilla-central/rev/1cc0a484ab9f