Modernize UI

[CSSFrancis]

Just some thoughts but things like:

1. Upsampling the icons to match the display
2. Adding a dark mode
3. Modernizing the UI
4. Making the starting logo transparent.

Would go a long way to making the HyperspyUI feel slightly more modern and that tends to make people feel like the project is maintained and worthwhile.

[ericpre]

Pyqt6 supports dark mode but the icons are not compatible with dark mode - they are dark on dark background!