BuddyPress.org

You could try:

#subnav ul li.current a {
    color: #e2e419;
    font-weight: bold;
}

Ref: https://developer.mozilla.org/en/docs/Web/CSS/font-weight

It’ll only change these links: #subnav ul li.current a. If you want it to change all links within the subnav list items and not just the current one, you can do this:

#subnav ul li a {
    color: #e2e419;
    font-weight: bold;
}