Dolphin already uses QStyleHints::colorScheme to detect dark mode,
so this code is now redundant.

bugfix:  SetQWidgetWindowDecorations(this); not called before show() for Windows darkmode titlebars.

The actual call to (QWidget) show() needed to come sooner. Show() was originally left alone, but with other checks needing to move with (QWidget) show(), this function became less useful.  Show()  was originally created to fix the render widget appearing behind the main window, but that appears to work fine in this iteration.

It's a fine function, but CPUThreadGuard is more vogue. Also, its potential for being confused with RunOnCPUThread will not be missed.

This makes it so that if you just want to reload the current style (eg. on program start, or in response to a system event), you don't need to know the name of the currently selected user style. It's also more consistent with the way the 'userstyle/enabled' flag works.