Theme Parts from twenty fifteen

Hello to ALL!

First, my English is not perfect. But i hope some people understand my problem.

I use the Theme great! But the this theme make same troubble with bbpress. The Answers will displayed in the top of Page, behind the Black Admin Bar. If i change the Theme to Twenty Fifteen it Looks fine! In the Documentation i read, i have to move a page to bbpess.php .. but it will not work .. can anyone tell me what i have to do. i only want the twenty Fifteen Forum Theme code use in my Great! Theme !

Thanks and greets!

IRoN