500 error internal server issue with WP theme

hi guys. i keep getting the 500 error internal server issue message whenever i try to customize my theme. the folks at one.com support, told me it was a problem with the old theme i was using and stupid me went and bought a new theme and now i keep getting the 500 error message everytime i try to customise the theme via widgets and he customise button. i have tried everything and i really need help fixing this!