add config option controlling which theme option(s) are available in the UI

How were the directories removed? the code doesn't remove the directories today so I suspect this is operator action.

Practically speaking, this should probably be something to the effect of 'enabled-themes' which identify which particular themes are enabled for the openstack dashboard.

Yeah I think it's about this logic:
https://github.com/openstack/charm-openstack-dashboard/blob/82897f8f5fde0ef2c639b18a2499d2f033ce37c5/templates/stein/local_settings.py#L912-L940

The quickest hack in this specific case is to set the custom theme as the default then "hide" the theme selection menu in the custom theme itself like the following:

$ git diff
diff --git a/custom-theme/static/_styles.scss b/custom-theme/static/_styles.scss
index e69de29..d474a4e 100644
--- a/custom-theme/static/_styles.scss
+++ b/custom-theme/static/_styles.scss
@@ -0,0 +1,3 @@
+.dropdown-menu.theme-picker {
+ display: none;
+}

It's not beautiful, but it does the trick.

Looking at the code, I have a clarifying question...

Is the user attempting to only allow their custom theme to be displayed? Or is the user interested in having the custom theme as the default theme and still allowing other themes?

Removing the themes from the filesystem is not really the right way to go about this, as a custom theme may depend on the default theme (and can indeed import pieces from that theme).

In this specific customer usecase, they would like to have the custom theme as the default and don't want to let users change the theme to other ones.

I agree that we shouldn't delete anything from the file system but to use AVAILABLE_THEMES to tweak the list of the themes to be displayed. In fact, the theme here has "@import "/themes/default/variables";" statement so if the default theme is deleted from the filesystem, Horizon gives 503.