Firebug is an excellent tool to find where you can change things. If you get Firefox and install Firebug (addon) you’ll get tons of work done. With it you can just point at things in the browser and it will tell you the current CSS that’s governing it. You can even add temporary CSS with Firebug to test things out.
The next step is to modify the CSS-changes in some file that you’re using. How proficient are you with that?
Look at:
The last line of style.css is: /* @import url( _inc/css/custom.css ); */. Remove the starting /* and the ending */ on that line. Save the file.
in:
https://codex.buddypress.org/how-to-guides/upgrading-a-buddypress-1-0-theme-for-buddypress-1-1/
In custom.css you can make the changes you want to add. If you’re not proficient with CSS you have to read up on that.
Firebug is a great tool. Saves you loads of time. Another way to acomplish what you want to do is search in _inc/css/screen.css for whatever you want to change and then do a replace with whatever you want to change it to. That’s the quick and dirty way!
Doing it Tore’s way with a custom.css is probably more fool-proof, cause you’ve got all your css changes in one place, but I prefer to work without a custom.css file…
I believe the font settings are all in the font css files. Logical enough 
The default theme uses the 12px sheet.
