Let’s face it, the default theme is ugly. Sure it was cool way back when, but now everyone has rounded corners and big blue headers. There may come a time when you would like to have the default theme on signup be something else.
For this, I am going to assume you’ve already picked another theme you like and tested it to make sure it works and behaves liek expected. Surf on in to your server, however you like. Find the default theme folder in wp-content/themes/. Tip: it’s called “default”. 
Rename the folder to something like default-old.
Now, find your new theme folder with the theme you like. Rename that folder to “default” (no quotes).
Seriously, that’s it. Withoin the code itself, it is only the folder name that is used in the case of new blog setup and to have a theme to revert to should a previously selected theme be removed.
If you want, you can disable Kubrick (the original default theme) so users cannot select it, by going to Site Admin -> Themes and click the “NO” radio button. Don’t forget to save your changes.
I’d also like to remind you to note that you made this change in your own personal changelog file. This helps by keeping track of any changes you made so you can repeat them in case of an upgrade, a re-install or some tragic unforseen event.
Hey, it can happen 
and it’s a good habit to get into.
Like most things “WP”, I didn’t know it was that easy.
It’s a good tip! But i’m trying to create my own theme, and to add some functions. It’s a good training !
And trying to have bbpress working too with the same theme is “la cerise sur le gateau” like we say in french
thanks, it’s simple light, speed, and cool !
Nice! I love it – one more question -
How can I have a theme enabled for my blog (the main blog), but not allow users to see it when they can select their themes? I don’t want them to have my theme…
You have to change the settings in the site admin settings. There is an option on the right hand side that allows you to make it in the available themes for the users.
I don’t see why we should even bother renaming the new theme to default.
Andrea,
Kudos, you seem to be all over when I am looking at tips, so you get another question.
How do I test a themes compatibility with WP MU to ensure that it will behave as expected ?
Is this a case of purely testing ? I’m looking at bpurchasing a custom theme, but have no experience on MU.
thanks
Paul
Yep, it’s mostly testing. Put it on your MU site, but enable it just for a single test blog. Then just activate it on that blog and give it a look-over. There’s no MU-specific template tags, so all you’re really looking for is plugin dependencies & hardcoded elements that users can’t change.
I’m sure I wrote about it in here somewhere.
Great site.. I have to say that I don´t know where to even start. But I am sure that I will find my way around here.
My biggest problem is that I get frustrated when I don’t see things happening right away.. and I am a hands on type of person meaning I leave the reading to only when I am stuck. That is if I don’t delete the whole thing..
This is awesome!!! it was so easy i was able to change the default blogs theme on my site.
Thanks a lot
How do you keep your users from having your default theme though? This is super easy, but I want to have my theme separate from the rest of them.
Thanks.
Same way as I note above – deactivate it under Site Admin -> Themes. This keeps it from showing up in the Appearance menu.
It will NOT change it on your blog.