Allows us to apply custom style overrides. Initially, this just
improves the appearance of disabled icons on dark themes.
The default Qt style method of displaying disabled icons only
works well on light backgrounds, on dark backgrounds it makes the
icons stand out due to the extreme contrast between the background
and the lightened icons.
Replace with a custom approach which desaturates icons and reduces
their opacity. This arguably also improves their appearance on
light themes too (I think so).
