We can reproduce this behaviour with our Theme Add-On and also other Theme Add-Ons.

You can find more information in the following ticket: https://jira.atlassian.com/browse/CONFSERVER-14838 (the fix of this issue causes this behaviour)

Below you can find the description and steps to reproduce the issue:

After disabling and Re-Enabling a theme add-on, the global site theme is set back to the "Confluence Default Theme" even if the site theme was
set to the installed custom theme. Steps to reproduce:

1. Install a Theme Add-On (for example: https://marketplace.atlassian.com/plugins/net.getunik.confluence.skin.core/server/overview or https://marketplace.atlassian.com/plugins/com.refinedwiki.confluence.plugins.theme.original/server/overview)

2. Activate the theme as site theme

3. Disable the installed Theme Add-On (Manage Add-Ons)

4. Enable the Theme Add-On (Manage Add-Ons)

Expected result: Global theme = Custom Theme Actual result: Global theme = Default Confluence Theme We haven't seen this behaviour in earlier versions of confluence.

This is quite a serious bug as lots of our customers use custom themes and the theme is set to the default theme, when for example the
confluence admin updates the UPM (as a result of updating the UPM all
add-ons are disabled and re-enabled). Users don't see the connection between updating the UPM and "loosing" the global theme.

Cheers
Stefi