For some reason the menu isn't getting compiled correctly. When its set as a mega menu, the links aren't supposed to have a defined width.
Try re-downloading the theme from themeforest and then uploading the theme files in the same way you first uploaded and installed the theme. Make a backup of any files you've directly edited in the theme so you can re-apply any changes later.
This will also updated you to 1.2 of the theme and it should fix the issue. If not, also try deactivating all plugins except woo commerce.
As a side note, if you ever want to make any css changes, add them to the Quick CSS in the Styling tab of the theme options or the custom.css file in the css folder of the theme files. That way you wont lose anything on updates and all changes are done without modifying the core files.