This is caused by the CSS reference for the \"active sort\" column. If you click on a different column heading (for example if you want to sort by Topic alphabetically), the column you clicked on becomes highlited with that light grey background. The top level of the forum doesn\'t have this problem because you can\'t sort on any of the columns on that page.
The fix is easy, for whoever controls the website.
In the file \"system.theme.css\", as part of the td.active tag, the background-color entry simply needs to be darkened up. Since the background color for the content DIV is #4e5460, I would suggest changing the td.active background color from #ddd to something more like #606870.