citra_qt: Use stylesheet with light colored icons if dark mode is used by the system

[ghost]

Fixes #520

Now the icon stylesheet isn't always black but dynamically changes depending on dark/light mode system state

TO:DO

Test Linux and macOS

Demo::

[OpenSauce04]

This PR doesn't seem to resolve the problem you're referring to, on Linux at least.
This is what the System theme looks like with Breeze Dark enabled in KDE Plasma 6:

I'm going to convert this PR to a draft. It can be marked as ready for review when it's working properly across the board.

[ghost]

@OpenSauce04 Can you try on macOS and let me know if it works there

I need to know if the issue is isolated to Linux. Most likely dark mode isn't being detected properly on Linux

[ghost]

Working fine on windows 11

[OpenSauce04]

Works on MacOS. Seems to be a Linux thing.

[ghost]

Fedora KDE 41
For some reason light mode doesn't use the stylesheet icons for me but system native ones instead however dark mode works

That's the way the normal build behaved gor me too

Screenshot (working fine on Fedora)

[OpenSauce04]

At some point in the next few days I'm going to spin up a Gentoo VM and install full-fat KDE to see if it's just my minimal KDE install being a bit weird.
If it's the latter we'll just progress with this PR.

[OpenSauce04]

Yeah that previous problem was totally just a skill issue on my part in some way, I tested using a Gentoo VM and it worked fine.
Will review now.

[OpenSauce04]

All looks fine to me

[OpenSauce04]

Merged as 420b803, e6ca711