The Similar Threads add-on uses default XenForo CSS classes. That way any style that is used will make the add-on look great.
In you case if you're not happy with the heading background color of the dark style you are using, I suggest adding an override in the EXTRA.css template. The class you want to change is:
Code:
.dataRow {
background:#ffffff;
}
Change #ffffff to any color you like.