Looking to change the width of my (actually non-compatible combination of Buddypress and Wordpress t

I've got this site I'm working on... and now, I am more or less a complete beginner on whatever and everything that has to do with Buddypress...

I've got a non-compatible Wordpress Theme, called Agility, fresh off the press, completely responsive, etc... great stuff. I'm trying to integrate Buddypress in this theme to get a restricted-access aspect going on. It seems to work pretty well, but I just can't seem to happen to be able to change the width of the Buddypress Plug-in...

Here is a screenshot of my test-page:


Any ideas?
And sorry, if this question has been asked so many times already, and if this question is actually a no-brainer!

Greetings from rainy Germany,