The same stylevar as above control threadtitle also:
font
If you want to change just that font and leave the others the same try this.
ACP -> Styles & Templates -> Search in Templates :
Code:
.threadbit .threadtitle {
That should give you a couple templates, choose:
threadlist.css
Find:
Code:
font-size: {vb:stylevar font.fontSize};
Replace with: Change the number in red to your desired size.