AFAIK: on Windows 10 there's no easy way to control the system colour palette other than registry hacks, or 3rd party payware.
I disabled the .qss tweak, and set the EnvVar thus:
Code:
SIGIL_FOCUS_HIGHLIGHT_COLOR=#FFFF00
That put a yellow box around the CV window ✔
But other places I still see midnight Blue, which is acceptable on text boxes, barely on buttons with text, and practically invisible on buttons with icons such as those under Preview.etc:
So I re-enabled the .qss tweak
Which gives me yellow borders everywhere ✔
BR