ACP -> Styles & Templates -> Style Manager -> Pick "Stylevars" from the dropdown menu of the style -> Search for:
font
This changes a number of fonts throughout the site. If you just want that Forum title font changed try this:
ACP -> Styles & Templates -> Search in Templates :
Code:
.forumbit_post .foruminfo .forumdata .forumtitle
That should give you a couple templates, choose:
forumbits.css
Find:
Code:
font-size: {vb:stylevar font.fontSize}px;
Replace with: The red "15" is the number you want to change to whatever suits your needs.