Account Activated page has a different theme

I am having a problem with the theme of my "Account Activated" page.

When a new user recieves the email asking them to click on the link, it takes them to a page that says "Account Activated" which is fine, but it's on a completely different theme, a theme that was set as my default theme for new users blogs in WPMU. But I have changed this setting and it persists.

I can't find the solution.

Any ideas?