Add Style for Steam Deck

[DavidoTek]

See https://github.com/DavidoTek/ProtonUp-Qt/discussions/309

• [X] Adds a fourth theme Steam with a Steam-like look
• [ ] Set this theme as default when Steam Deck is detected
• [X] Revise dialogs to not use a fixed size (will break the theme otherwise) -> fixed with https://github.com/DavidoTek/ProtonUp-Qt/pull/396
• [ ] (Future PR?) Automatically resize dialogs when Steam Deck is detected