Custom palette and style sheet for Qt UI light and dark mode, with live update

[Zueuk]

Summary

Custom palette and style sheet for Qt UI, both light and dark mode for the Fusion theme, using the classic OneTrainer colors.

Test plan

• pre-commit run --all-files passes
• Launched the affected UI or script and exercised the change
• Tested with at least one real preset / config when relevant (not relevant - UI only)

AI assistance

• slightly AI-assisted — I have read every line in this diff and can defend each change

[dxqb]

can't test dark mode, there is no dark mode on my platform. I'm okay with a windows-only dark mode PR, but somebody else should test.

Regarding light mode in this PR:

I'm not sure we should make dropdowns and buttons blue just because Ctk had them blue. doesn't look better to me than standard Fusion

[Zueuk]

Dunno, I think the blue accent on buttons & etc is a nice touch that makes the UI much less boring... And familiar, for long time users.

Also, checkboxes are much more visible with the blue accent.

[dxqb]

please coordinate with #1488