BuddyPress.org

First install Firebug in Firefox:
http://getfirebug.com/

Then use it to view the css you want to copy, then adjust your css accordingly.

BTW: The buttons used on this site are created in part by css3 (roundness) . CSS3 does not work in any version of Internet Explorer.

Also make sure to add all the browser selectors (-moz, -webkit, etc) as a precautionary step due to the young age of these CSS3 properties. This will insure it will look correct in all browsers (except of course IE).

For example, in Firebug the css may look like this:

a.button, input.button, div.generic-button a {
-moz-border-radius:13px 13px 13px 13px;
}

But you will need to add the other selector so it looks like this:

a.button, input.button, div.generic-button a {
-moz-border-radius:13px 13px 13px 13px;
-webkit-border-radius:13px 13px 13px 13px;
border-radius:13px 13px 13px 13px;
}