Horizon supports the use of multiple concurrent themes. Describe how Horizon themes can be managed. An example of customizing a theme would be helpful.
It looks like this feature is already described here: https://access.redhat.com/documentation/en/red-hat-openstack-platform/9/paged/introduction-to-the-openstack-dashboard/chapter-5-customizing-the-dashboard In particular: "To define custom SCSS files that apply only to a specific dashboard, create a base template in your dashboard’s templates folder, which extends the dashboard’s base template [...]"