You can use svn, but you want have commit access (http://svn.buddypress.org). As for reporting bugs, and viewing what needs to be fixed, you can do that on Trac (http://trac.buddypress.org). You’ll use the same login credentials to post on Trac that you use to post here.
Anyone is able to create a trac ticket for bug notifications or enhancements once created it will await review by one of the three core commiters (those three + 1 are the only ones with commit access on the project), if approved or passed as valid others that actively run trunk/bleeding edge test sites might look at and if necessary feedback and/or test patches as required.
If you have enhancements / code changes then submit them as patch files that can be tested out on installs.
i’d like to contribute by remodeling the default theme 
Do it then but not the default theme, create a child theme which is essentially re-modeling the default. Themes are in short supply so if you have the ability then I’m sure many will be grateful for an addition to the theme library for BP.
Cool. What format should patch files be in? Unified diff format?
Yep. ‘svn diff > patchfile’ I what I usually do.
Trunk is where the dev is happening, so if you’ve spotted things you want to fix, check out a trunk install and check that we haven’t fixed it already.
If you’re looking for things to fix, typically any ticket on trac is valid and warrants a patch, but see if you think you can attack any of the issues in this report. These are the main tickets we still would like to fix for the next release: https://trac.buddypress.org/report/11
Thanks
Regarding the theme: if you want to make your own version, do as hnla suggests. Otherwise, how about mocking up some designs for post formats support and see how things go?
just another BP child theme?
no man 
all those child themes look the same,can’t believe people waste their time on developing that kind of things.
The default theme needs improvements as well,lot’s of things makes no sense to me and it’s kinda messy.
Especially activity stream.
You will find that currently as things stand all themes for BP are in effect child themes, themeing for BP is not quite the same as themeing for WP. I am presently working through a new theme or at least framework it replaces all theme files , referencing only the JS files in essence, yet many of the files will remain structurally pretty similar, but that is the framework for then tweaking the UX and UI to something that one prefers, then you start to have to get involved in filtering actions and playing around with template tags where you are able.
This is as different as one is able to take a BP theme I tend not to think of it as a child theme or a re-skining but in truth it is technically a child theme if one was to be accurate about it.
The bp default theme is probably past it’s sell by date somewhat and there has been vague talk about setting to and developing the next generation theme so give it a go.
You can do what you, but I think there’s not much scope other than post formats for any more changes to the theme on the upcoming release cycle. If you want to prove me wrong, I suggest making some mockups before you start writing code.
No,I don’t want to prove you wrong,it’s just my opinion.
few things I’d like to change :
if you’re looking at your own profile then you should see friends activity as default instead of your own,makes more sense to me.
comments on your own activity should not repeat as activity in your own stream.
allow users to delete and filter comments and @ mentions on their profile. that way users can delete unwanted comments and @ mentions (SPAM)
if user is not your friend then he should not be able to comment on your activity stream.
and of course privacy settings are very important.
Few other cool things are ,you may know this user and birthday notifications.
Child themes don’t restrict anything. You can have a child theme that completely overwrites the parent theme’s code. The idea behind a child theme is that it allows uniformity, and something for plugin authors to count on when it comes to hooks and such.
Also, most of the changes you’d like to see are changes that would need to happen in core, with minimal code changes to the theme, and a few of them would be doable as plugins.
I agree that the default theme needs a designers touch, but it has all the nuts and bolts to make everything work. If for some reason your new theme breaks, the default will at least be there to back you up. Plus its always helpful to be able to simply turn off your theme to see if its a BP issue or a issue in something you did.. If it works in default then it will work in your theme.
What your talking about with the stream I agree changes should be made, but thats a personal preference so you have to write that function into whatever you are doing and go from there..
