BuddyPress.org

When you say tweak it all depends on what you want to do as a lot you can simply do using CSS which means no needs to dive into the actual templates. I’d recommend a tool like Firebug http://getfirebug.com/ or Inspector depending on what browser you use. Using that you can view your page and inspect each element adjusting the CSS live and see the results.

Now, if you do want to dive deeper into templates it all depends on your theme. What should happen is your theme would be a child of BP Default ‘or’ use the template pack – I don’t know what theme you are using so don’t know what is the case. Some premium themes do bundle the files for BuddyPress (and often BBPress) in with the theme. Depending on your theme does also depend on where the BBPress template files may be. It could be it’s using default styling, it could be it has called it’s own – in this case the files would be in your theme folder. Ideally it should do the later if using customisation.

If you just end up doing CSS you can override in your themes CSS.

I would encourage you if you can to link in your site as it’s not the best way to view an image and diagnose CSS (or suggest if just can be CSS). Also stating what you want beyond you don’t like look – most things can be done in CSS but you may want more structural and for that it would be wrong to just suggest CSS.