Client dark mode support

[RokeJulianLockhart]

Is your feature request related to a problem? Please describe.

I have applied a dark system theme via the undermentioned methods:

• #!/usr/bin/env -S sh
  # kcmshell6 --list
  kcmshell6

• systemsettings

• #!/usr/bin/env -S sh
  # lookandfeeltool --list
  lookandfeeltool --apply 'org.kde.breezedark.desktop'

...as demonstrated (by the undermentioned screenshot):

...yet TigerVNC remains unaffected by this (per the undermentioned screenshot):

...which is in stark contrast to everything else I've ever used.

Describe the solution you'd like

Adhere to system theme preferences.

Describe alternatives you've considered

At the very least, provide a custom theming system so that I can maintain one that replaces Breeze.

[CendioOssman]

This is unfortunately more of an issue of the @FLTK GUI toolkit we use, which currently has no dark mode support (and not much in the way of theming either). So it likely requires some new features there first.

But we can have a tracker issue here to keep an eye on it from our end.

[timothytylee]

This is unfortunately more of an issue of the @fltk GUI toolkit we use, which currently has no dark mode support (and not much in the way of theming either). So it likely requires some new features there first.

But we can have a tracker issue here to keep an eye on it from our end.

I've created PR #1846 which attempts to implement dark mode in FLTK.

[RokeJulianLockhart]

https://github.com/fltk/fltk/issues/870#issue-2045240052 appears to track fixing this upstream.

However, since all the issues like https://github.com/fltk/fltk/issues/972#event-13342500332 that I've seen – about adherence to system colouration, irrespective of darkness – appear solved, I'm surprised TigerVNC doesn't adhere by default.