[Feature] Add option for Dark mode

[TK50P]

Is your feature request related to a problem? Please describe.
No. however, I use with Rectify11. that breaks some elements.

Describe the solution you'd like
Please add the option of Dark theme in settings.

Describe alternatives you've considered
None.

Additional context

[daviunic]

The changes for BackColor that fix these problems on dark mode are already in since commit f7bc5ae, I just have to wrap up all the changes and release 1.7.5.

[PalZer0]

Going to bump this up as 86Box build 6540 has introduced dark mode and it would be nice to have 86Box Manager follow suit with a new release (given the code is already ready to go).