Header logo problems

Here is my problem.
I chose a theme and tried to change my header image logo (the logo on top)
I successfully changed it and save my settings (works fine on the preview at the bottom)
However, i went to preview (top left) my forum and it screwed up.
The header logo works fine but the rest of it, the colour scheme of the forum and stuff were totally wrong.
Is there a way to solve this?