### Purpose
Fixes#9776 by tweaking the text/background colours of disabled checkbox
panels when dark mode is enabled.
It was [noted on that
issue](https://github.com/syncthing/syncthing/issues/9776#issuecomment-2424828520)
that there's a bigger issue around the correctness of using the
`disabled` attribute on a `<div>` in the first place, but this PR does
not attempt to change that.
### Testing
I've hooked up the GUI files against a release build as suggested below.
### Screenshots
Using the dark theme, or the default theme with a system dark scheme:
Using the black theme:
These borrow the colours from dark theme text inputs and black theme
tabs for a consistent look (initially I tried the text colour of
disabled text inputs, but that produced some poor contrast).
