include 'background_color' in list of supported keys

[doubleO8]

• Resolves: [Bug]: theming:config background_color neither shown nor configurable #51182 51181

[susnux]

@doubleO8 thank you for your pull request. Could you please sign off your commit as per our contribution guidelines?
Should be as simple as git commit --amend -s.

Also please adjust your commit message to follow the conventional commits, e.g. simply:

- include 'background_color' in list of supported keys
+ fix(theming): include `background_color` in list of supported keys

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