gui: fix custom color support UI rendering issues

[fsagbuya]

ARTIQ Pull Request

Description of Changes

Fixes issues with custom color support interfering with normal widget styling:

• Handle title bar and window colors independently
• Enhanced reset colors to properly restore default Qt/UI settings
• Refactored gradient handling to preserve appearance with custom colors
• Replaced styleSheet with palette for consistent widget styling

Default view on Windows:

Default view on Linux (gradient is lighter in pyqt6)

Colored View on Linux:

Related Issue

Closes #2661

Type of Changes

	Type
✓	🐛 Bug fix
✓	🔨 Refactoring

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  - Tested on Linux and Windows

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[sbourdeauducq]

Generally seems a bit messy.