That looks very much like your WordPress theme styling is having quite a marked effect on the SP theme styling. This is something that can not be automatically catered for simply because there are thousands of WP themes which are all different!
Go to the forum admin > themes > theme editor - open it and select the template file spHead.php.
Scroll down a few lines until you find the line of code that is:
and then immediately AFTER that line of code add the following line:
EXACTLY as shown. See if that helps.
Yes that looks correct. Is there any chance that you could briefly (just to test) switch your WordPress theme to one of the default themes they supply to see if this still happens. Switch straight back to your own theme afterwards of course.
and after that let me know and if possible, give me a link to your forum page.