bug: some icons in the quickemu dark theme have incorrect contrast

[flexiondotorg]

The colours for the dark theme need some improvements, so the active and inactive state is clearly communicated.

[andypiper]

I just installed quickgui and went to enable dark mode (the correct mode), but found that the option is disabled. The reason was not completely clear until I found this issue and the associated commit. It may be better to remove the option rather than disabling it, to avoid similar confusion.

(the best approach would be to improve the colours or whatever else is required here; and then to follow the system light/dark preference).

[ivoheck]

I use QuickGUI regularly and was wondering why the dark mode switch wasn’t working. I saw this issue and would like to work on it. I'm new to open-source contributions—can I go ahead and fix this and open a PR? Let me know if there's anything specific I should keep in mind!