Add dark theme or custom theme support #6748

Is your feature request related to a problem? Please describe.
I am not sure if the removal of dark theme in 6.09 was intentional, although given how buggy the GTK+ option was (the only dark theme), it would make sense for it to be intentionally removed

Describe the solution you'd like
Add an option to enable dark theme, preferably one that uses GTK, Qt or customizable by other means, but I would still greatly appreciate a single option

Additional context
There may already be dark theme support for Windows and/or MacOS (which would explain the lack of issues requesting this), but currently there is no option for dark theme for Linux