BuddyPress

As the title said, how can i let the user of my site that creates a blog upload and use their own theme?..

I’m going to assume you mean under WordPress not BuddyPress specifically and using Multisite? Is that right? If not what do you mean by theme?

Yes, the multisite where users creates their own blogs.. i would like to allow them to upload themes for their own blog.

I believe you would have to give them some administrative rights. Try one of the role defining plugins that let you dictate EXACTLY what each user role is capable of. Try “Capability Manager”, if i recall it is pretty good.

For multisite install, only Network Admin can upload and enable themes for activation in different blogs. This is to provide a level of security for the whole network. If you allow users to upload themes, your installation will surely be hacked.

So that is out of the question. Lets say if i get a person that is really putting effort into their blog. I Would like have it where when i fill like they are a real asset to our website, i can contact them requesting if they had their own theme they would like to use with their blog and if they do, they can send me the theme and i can add it so it will only be available to their site and no one else?

Yes. If a user bought a premium theme or created his own theme, the user can email the theme to you and you can upload and activate it for use in that specific user’s blog only. No problem.

Note however that it is the responsibility of Network Admin to check out that theme first for security holes, deprecated tags, etc. before uploading it to server.