Comment 2 for bug 1960358

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;
+}