Well, it's still going to be some time before I switch Rufus UI to XAML Islands and (hopefully) get Dark Mode automatically, so, if you want to invest your time to add Dark Mode to the current GDI-based UI, and considering that I do expect to be pestered by people who want Dark Mode for quite a few more years, I definitely wouldn't mind having an intermediate solution for Rufus.

This means that, if you are willing to contribute one, I'm not going to shut you down, as long as it doesn't look like it's going to be too intrusive to the existing code, which, at first glance at your draft proposal, doesn't look like is it, and as long as you fix the MinGW build issues. So you are getting a thumbs up from me.

If it helps, I'm also going to attach the patch that I originally played with to produce my own version of dark themed Rufus, which produced the output you see below and that I posted in #1453 (Unlike you, I didn't get to sort the progress bar and backgrounds, but I did sort out the title bar, so maybe you'll find something useful, though I have to point out that this was a quick & dirty tryout and definitely not production-ready code):

For the record, this patch is meant to be applied on top of commit 129aa64, which means it was designed circa Rufus 3.10, so, obviously, the code has changed a bit since then.