Feature request: Add an option to put image background under Settings/Add-ons page sidebar

### Is your feature request related to a problem?

I think, that there should be an option to make the Settings page/Add-ons page sidebar have a background image instead of a monochrome colour background (white/black).

The only problem would be, that users would manually measure the height of the sidebar since everyone has different toolbars settings (bookmarks bar, title bar or third-party toolbars) which would make the height of the sidebar image dynamic, but that is a problem for users themselves and not for the actual implementation I think.

### The solution I'd like

Add an option to the user.js/about:config to allow the settings sidebar to use a background image for better customisation.

### Screenshots or video

#### Affected sidebars:

Settings page sidebar:

Add-ons page sidebar:

### Alternatives I've considered

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating the SCSS that styles the Firefox Settings and Add-ons sidebars, then check how user.js/about:config options are defined. Done means an option can enable a background image for both affected sidebars without changing their default monochrome backgrounds.

Written by the indexing model from the issue text.