Re: Module Font Fivealive
No, div class="rbox-data" (or .rbox-data as it appears in the stylesheets) is something else (the notice boxes that appear in the center column usually). The modules use div class="box" (and their content is in the "box-data" div within the "box" div). The syntax in CSS files for a class is to precede the name with a dot, so in this case the selector in the stylesheet is ".box .box-data".
Which CSS file you modify depends on where you want the change to appear. The theme option stylesheets will normally override the main stylesheet for any rule they both contain. Or you could just put the CSS rule in the custom CSS textarea of the Look and Feel admin section, and it would be used (but will affect all themes, which could be an issue if a site uses more than one).
I'm not sure offhand what the current font size setting is for .box-data in the Fivealive theme (maybe 12px), but once you find that, you could just ratchet it up a little, either using a pixel size or a percentage. And use the CSS syntax for bold. Maybe you should check out some CSS references if you're not sure. http://www.w3schools.com/css/ is a good one.
-- Gary