I had the same problem, and I fixed it by editing the following CSS classes (..\DesktopModules\Forum\Themes\Default\default.css):
TD.Forum_Row_Alt
{
background-color: #f2f3f8;
border-top: #cfd2d7 1px solid;
border-left: solid 1px #cfd2d7;
}
/* Threads Column in initial group view */
TD.Forum_RowHighlight1, .Forum_RowHighLight1
{
background-color: #E4E6EC;
border-left: solid 1px #cfd2d7;
border-top: #cfd2d7 1px solid;
}
/* Threads Column in initial group view (odd)*/
TD.Forum_RowHighlight1_Alt, .Forum_RowHighLight1_Alt
{
background-color: #f2f3f8;
border-left: solid 1px #cfd2d7;
border-top: #cfd2d7 1px solid;
}
/* Posts Column in initial group view */
TD.Forum_RowHighlight2, .Forum_RowHighLight2
{
background-color: #E4E6EC;
border-left: solid 1px #cfd2d7;
border-top: #cfd2d7 1px solid;
}
/* Posts Column in initial group view (odd)*/
TD.Forum_RowHighlight2_Alt, .Forum_RowHighLight2_Alt
{
background-color: #f2f3f8;
border-left: solid 1px #cfd2d7;
border-top: #cfd2d7 1px solid;
}
/* Last Post Column in initial group view */
TD.Forum_RowHighlight3, .Forum_RowHighLight3
{
background-color: #E4E6EC;
border-left: solid 1px #cfd2d7;
border-top: #cfd2d7 1px solid;
border-right: solid 1px #cfd2d7;
}
/* Last Post Column in initial group view *(odd) */
TD.Forum_RowHighlight3_Alt, .Forum_RowHighLight3_Alt
{
background-color: #f2f3f8;
border-left: solid 1px #cfd2d7;
border-top: #cfd2d7 1px solid;
border-right: solid 1px #cfd2d7;
}
You can copy the code above and change the background and border colors to fit your needs. I hope it will solve your problem.