It would be great to have this feature merged; like many organisations, we share a single realm for multiple clients, and we would like to customise the emails as we do for the login.

I can see that tests are missing on this particular MR; looking at the code, there seem to be no tests related to the DefaultThemeSelectorProvider, email and email themes. What's the expected extent of testing for this MR? Once fully tested, would a merge be considered?

An email is not always sent in the context of a client, so it makes little sense to me to have a theme per client. To be perfectly honest I've also been against the concept of having a theme per client, and think there are better ways to achieve that. However, there's not sufficient details in the JIRA or here to better understand the use-case.

I would suggest if this is still wanted to open a discussion around it.