Fix a few minor issues with the new icon themes, and Qt6 theme support

[vkbo]

Summary:

This PR:

• Updates the calculation of the different shade colours, which are based in the Window colour. If the Window colour is black, all shades are also black, rendering some widgets invisible.
• Update how icon themes are handled. They are now consistent with other themes and have proper default fallbacks defined. Otherwise the icons would fall back to all red X-es.
• Add support for user defined icon themes, as requested by @HeyMyian. The new themes are more complicated to make than the old one, but at least now there's the option. The only real difference is that the icons must now be SVG and must be in-lined in the theme file rather than be saved in individual SVG files.
• Status LED colours are now taken from the default, red and green icon colours from the theme, and made a little darker.
• The Solarized themes have been corrected a little based on the parent theme standard.
• The status led colours have been removed from all themes, and a few minor adjustments made here and there.
• The custom toggle switch has been modified a little. The thumb button is now always highlightedText colour, and the track uses alternateBase colour when off. It has also received a border using one of the button shades. All of this should improve visibility on all themes.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed