I have the same issue originally posted.
I downloaded and installed the latest code. I added a custom theme, which works perfectly. My only issue is that the theme randomly changes for some users.
My first blog is utilizing my custom theme and working great. I created another blog yesterday and issued the Default theme. When I made this change to the second blog, the theme for the first blog changed.
In the setting for the first blog, I have the default theme set to my custom theme. Why is this setting not working? I have removed the drop down control which gives access to change the theme, but the setting is still true for allowing users to change the
theme. I kept this setting due to the fact the theme randomly switches.
Thanks for all you help! Any feedback would be great!