I always add stuff to additional.css. But, sorry, that was for the forumdisplay.php page and you want it on forum.php. I grabbed the wrong code:
HTML Code:
/* Increase last post by column on forum page - make sure add to 90% */
.forumbit_nopost .forumhead .forumtitle { min-width: 66%; }
.forumbit_post .foruminfo {min-width:30%;
width:47%;}
.forumbit_nopost .forumhead span.forumlastpost, .forumbit_post .forumhead span.forumlastpost, .forumbit_post .forumrow .forumlastpost {width:32%;}
.forumbit_post .forumstats, .forumbit_post .forumstats_2 {width:13%;}