Qt: use default colors for native styles

[Megamouse]

Fixes several color issues when using native styles:

• Sets icon background colors to transparent
• Uses the native styles' label foreground color for any regular texts
• Uses the native styles' label foreground color for toolbar icons
• Uses the native styles' label foreground color for the gamepad image in the pad settings
• Uses green/red color some patch creator labels
• Uses blue/brightblue link color depending on dark mode (ironically link colors are broken in Qt 6.6.0)
• Uses log colors inspired by existing stylesheets depending on dark mode
• Repaints the UI when the OS switches between light and dark mode

Fusion: