feat: Custom background gradient support

[LilyLavenderPony]

Description

This PR introduces a customizable background gradient feature, allowing users to define up to five colors, gradient angle, transparency and the option to apply the gradient on background elements only or the whole screen.

The gradient is applied on top of existing themes rather than introducing new theme variants, enabling lightweight visual customization without affecting the core theming system.

Examples using the Silver and Butter themes are shown below:

Implementation

• Introduced CustomThemeManager for dynamic background styling
• Added color pickers and controls in Appearance settings
• Gradient is computed and applied at runtime based on user settings
• Added a separate div the gradient is applied on to not interfere with existing architecture and projects of other contributors

Design considerations

This proposal builds on the discussion in #357 with some differences:

• Blur support is implemented but currently not exposed in the UI, as its visual impact is minimal in the current layout. This can be revisited in future iterations.
• No additional structural class hooks were introduced. The implementation avoids relying on internal layout details, reducing maintenance overhead and improving compatibility with future UI changes.

How to test

1. • Enable "Custom Background" in settings
2. • (De-)select multiple colors
3. • Adjust angle and transparency
4. • Disable the setting and confirm it resets as intended

Tested on:

• Firefox
• Chrome

[github-actions]

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[LilyLavenderPony]

I have read the CLA Document and I hereby sign the CLA

[u9000-Nine]

So adding a separate div on top isn't really the same thing as changing the background. This is especially apparent in the second image, where the gradient visibly tints all icons and text.

Also, if we're going to start adding theme options, then "Appearance" should be its own settings tab instead of dominating General.

[LilyLavenderPony]

Also, if we're going to start adding theme options, then "Appearance" should be its own settings tab instead of dominating General.

You are right, this is a good point.

So adding a separate div on top isn't really the same thing as changing the background. This is especially apparent in the second image, where the gradient visibly tints all icons and text.

That is true. I chose this approach because modifying the actual background would require changes that could conflict with this PR, which is currently reworking the styling. So I went with a more isolated solution for now.

However, I agree that the tinting of icons and text is unexpected behaviour for users and results in poor UX. It may be better to revisit or rework this PR once the CSS changes in Cinny have stabilized.

Thanks to you and @kfiven for the feedback, and for moving this PR to draft.