Add user.css settings

[ericgaspar]

• • Add custom CSS setting with user.css*

PR Status

• [ ] Code finished.
• [ ] Tested with Package_check.
• [ ] Fix or enhancement tested.
• [ ] Upgrade from last version tested.
• [ ] Can be reviewed and tested.

Package_check results

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If you have access to App Continuous Integration for packagers you can provide a link to the package_check results like below, replacing '-NUM-' in this link by the PR number and USERNAME by your username on the ci-apps-dev. Or you provide a screenshot or a pastebin of the results