Custom Background Color Selection #98

Description

Add an option to customize the AuroraShell background/panel color, preferably alongside the existing accent color configuration.

Currently, the gray background is part of AuroraShell's visual design, but some users may prefer a different background depending on their desktop theme, wallpaper, monitor, or personal preference.

Providing a simple background color selector would allow users to personalize AuroraShell while keeping the default appearance unchanged for users who prefer it.

Expected Behavior

Add a background color option to AuroraShell settings that allows users to:

• Select a custom background/panel color.
• Easily reset the color to the AuroraShell default.
• Preview or immediately apply the selected color.
• Keep the existing gray color as the default.

This could be placed near the existing accent color configuration for consistency.

Additional Context

This would also complement future visual features such as blur/background effects and could provide more flexibility for OLED-oriented themes using pure black surfaces.