Add Style for Steam Deck

[DavidoTek]

See #309

• Adds a fourth theme steam with a Steam-like look
• Set this theme as default when Steam Deck is detected (system theme as default otherwise)
• Revise dialogs to not use a fixed size (will break the theme otherwise) -> fixed with Make Dialogs resizable #396
  • Resize the custom install dialog to fit the steam theme
• (Future PR?) Automatically resize dialogs when Steam Deck is detected

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Regarding the global stylesheet:

My experience with Qt stylesheets is that once you start going too global, the stylesheet's explicitness needs to increase. Missing values from the stylesheet will start overriding Fusion to use Qt defaults, making for a very weird and ugly UI in some cases. - #487 (comment)