When using Qt's Fusion style on Windows, setting the colour scheme in Qt can override the system colour scheme, so you can get a dark palette in LOOT when the Windows colour scheme is light, and vice versa.

However, that isn't the case for GTK and KDE themes on Linux, which seem to basically ignore the colour scheme you set if it doesn't match the current theme's colour scheme: see the Qt code for GTK and KDE.

I guess this is because the Fusion style on Windows is almost completely bypassing the actual Windows theme (aside from its accent colour, I think), while on Linux it does still try to match the system theme, and the system theme doesn't define light and dark colour schemes, instead there are two separate light and dark system themes.

This means that trying to use LOOT's default theme variant that's the opposite of the system colour scheme gives the system colour scheme with the opposite custom colours from LOOT's stylesheets, which looks bad and makes some text unreadable.

I tested a whole bunch of combinations: