I have the same question. I’d like to add a bit to the standard template then have that template be the default template for people who create a blog. So, they will be able to make posts but not change the template.
Trying to accomplish the same objective, I found this article: https://mu.wordpress.org/forums/topic/1463
After following the setup directions, when I created a new user, created a blog, created a post, and then went to view the post, it appeared in the expected theme. As the user, I *did not* need to go to set the theme myself.
Towards the end of the thread, there is the following
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As of 2.7 this information:
….
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Lines 311 and 312 of wp-admin/includes/schema.php in the populate_options function.
add_option(‘template’, ‘yourthemename’);
add_option(‘stylesheet’, ‘yourthemename’);
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For those that want to use this solution with a theme like ‘default’ or ‘classic’, I think this is enough to get you going. For me, I find it a bit discontinuous to start from the buddypress theme home page and then bounce into the ‘classic’ theme of some individual user, and still think about it all as being the same site. I completely respect that that is a legitimate model & approach. It’s just not what I have in mind for my particular implementation.
Unfortunately, it’s not quite enough to just make the facebuddy theme default for all users. The ‘thing’ I want to ‘resolve’ is that for any user’s new blog, wordpress basically gives that user their own buddypress front page, that the user then needs to configure with their own widgets and stuff through the wordpress admin.
For my needs, I would like to make something that is more turnkey for new users, such that they only have to register & create the blog & start posting. I don’t want them to have to think about layout. I may offer them the option, but I want their blog home page to look polished off the bat. Furthermore, this ideal default setup would have a unified feel with the entire site so that as you went from user blog to user blog, a visitor feels like all the content look & feel is homogenous across all blogs in the wpressMU install.
Another way to put this idea is … when you go to the user’s specific blog page, there should more emphasis on the fact that it is a user of the main site discussing on some specific site topic, within the context of the site branding, etc …. rather than making the distinction that you are visiting someone else’s blog. Like a newspaper … only the by-line and the article content vary.
I have some ideas on ways I can approach this. If anyone else is interested in this, I’d be happy to share ideas and collaborate on knocking out a solution. Maybe someone has already solved this problem, but if they haven’t, I imagine it could be useful to a few other folks as well.
BuddyPress is just Awesome!
Whatever theme you want to be the default for your blogs needs to be put in the ‘default’ folder in the wp-content > themes.
If you want to use the buddypress home theme for the blogs copy it (with your customizations) into the default folder. The folder needs to stay named Default. When WPMU/BP creates a blog, it will use whatever the Default theme is.
All the best to you!
hmm … that’s a lot more straight forward than what i found. cool 
Regarding my ‘thinking out-loud’ point above about figuring out how to implement a consolidated theme across all buddypress blogs and the main page …
This morning, as I was clicking on the CNN site, and I noticed they have a bunch of different individual & topical blogs. I wanted to see how they unified the look and feel, so I clicked on several different ones. What they have there is just a simplified layout of their main site … essentially, a subset of the main page layout. So it (finally) occurred to me that what I can do for this is
1. Figure out template setup basics, enough to …
2. Put together a basic template that uses the same color, font, and graphical components from the site’s main theme, but is a very basic theme, not one that is buddypress ready. It will look like it is the same theme, but that will be a deliberate coincidence.
* I can hardcode widget and/or custom code into there as necessary
* I imagine this part can get tricky, but it does some reasonable to do.
3. Make this template the default for all new users
I acknowledge this is probably not such a revelation to many of the WPress initiated … but for me, it took some time and mental crunching to get to this conclusion. From my humble newbie vantage point , it does seem like it could be a useful consideration if users have their own blogs.
