yuzu's default theme doesn't specify everything, which is fine for
windows, but in linux anything unspecified is set to the users theme.

Symptoms of this are that a linux user with a dark theme won't think
to change the theme to a dark theme when first using yuzu

Idea here is to try and support arbitrary themes on linux.

preliminary work on a "default_dark" theme, used only as overlay
for any themes that are measured to be dark mode.

Other work done:

FreeDesktop standard icon names:
plus -> list-add
delete refresh, we use view-refresh

remove duplicated icons for qdarkstyle_midnight_blue
referencing icon aliases in the qrc files is the way to go

Note:
Dynamic style changing doesn't appear to work with AppImage