feat: set background = backgroundColor via occ

[kesselb]

Summary

Before:

$ occ theming:config background backgroundColor
The image file needs to be provided as an absolute path: backgroundColor.

After:

$ occ theming:config background backgroundColor       
Updated backgroundMime to backgroundColor

TODO

• CI

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[skjnldsv]

This feels a bit undiscoverable, but fair enough :)

[kesselb]

This feels a bit undiscoverable, but fair enough :)

True ;)

Sorry, I forgot to mention the pull request to update the documentation: nextcloud/documentation#10569

I will send another to document occ theming:config background backgroundColor after merge.