Changing 'Modern Style' post width

I’m trying to modify the theme “Modern Style” (http://flexithemes.com/themes/modern-style/) for a new project. The videos I post are a little wide for the theme and the video touches the navigation box.

My problem is I just can’t find any definition of width in the theme files, there is just no value in the sixhundreds except of maximum image width. What would be the right point to add this definition to the theme?

Contacting the creator won’t help – they like to sell subscriptions for support. 😉

Thanks for any help.