A year ago or so I had a setup where my chromium would be all uniform white/black when unfocused and the tabs would turn gray when focused, there was no need for any borders with that. I used the ChromeOS gtk theme because it had a distinct blue tab colors by default, I looked the blue up and edited the files, I just tried to quickly replicate it, and I more or less could, in brave, but only with the gtk buttons idk, mine worked without them, ask someone who actually knows gtk or go for trial and error like I did, but here:

Line 1861+ of ChromeOS/gtk-3.0 /********* * Menus * *********/ menubar, .menubar { -GtkWidget-window-dragging: true; padding: 0; background-color: #00ff00; color: white; }

menubar:backdrop,
.menubar:backdrop {
  background-color: #ff0000;
  color: rgba(255, 255, 255, 0.7);
}

This is editing generic menus I guess, later Chromium gets specified more I bet you could edit just that.