CSS is theme specific. If you have custom CSS and later change themes, you need to delete whatever CSS you have in the Editor, otherwise, you’ll get undesired results. To ‘reset’ a theme, you need to delete the CSS in the Editor, make sure the “add to existing CSS” radio button is checked, then save. Things should go back to normal.
If you are not successful when searching there is a link for the specific Premium Theme support forum for your theme under Appearance -> Themes in your blog’s Dashboard, but do note that the premium theme must be active and you must be signed in as the user who purchased it in order to see it and post to it.