Customise UI colours in the Brave window

Is there any way to customise the colours of the Brave window/UI? I see in the browser settings that I can pick a “theme” from the Chrome store but most of these themes are about bringing a large photo into the browser. None of that is even visible anywhere in Brave. The tab bar area doesn’t show a photo and the new tab page brings its own photo. So I don’t see any use in that part of the browser settings UI. Is it a broken leftover from another age?

Anyway, I don’t want photos in my browser UI. One of the themes actually did change the tab bar background in a way that was different for the active and inactive window. But it introduced gradients and other unwanted colour changes.

I want to customise a few things:

  • The background colour of the tab bar. And I need it to be different for active and inactive windows. (I’m not talking about a 2% change in grey lightness, but a difference like grey vs. blue.) It seems to be possible but a ready solution is not provided. So how can I fix this and make my own solution? How can I create such a “theme” and use it in Brave? It looks like a closed system that is not intended to be extended by users.
  • The tab header colours also need to be changed. The contrast between active and inactive tab is too low. And the text colour of an inactive tab is too weak. To make any use of that tab bar, I’m supposed to pick an inactive tab. But how can I pick a tab if I can hardly see which one it is? Some designer has probably forgotten the usability of the product when they made that.
  • Is it also possible to replace the toolbar icons? Firefox has just released new icons and I rather like those. (I don’t like their new “tab header” design though, it can be disabled for now but leaves other glitches.) Those triangle outlines for the back and forward buttons look quite ugly to me. I prefer normal arrows. So can they be replaced or is that ugliness an integral part of Brave on Windows?

If none of the above is possible, where can I find instructions to set up a build environment for the complete browser package, including an estimation about how long it takes to set things up and to actually build the browser from source on a Core i7 from 2012?