Constrain Header Color to Container

[taylor_smith]

Greetings! I come as a humble, non-techy to seek the wisdom of the demigods.

How do I constrain a header color to my container width? The site is here: http://www.carpediem.im/fora. Alternatively, see image below.

Thank you for your insight.

 

[HenrikHansen]

http://xenforo.com/community/threads/header-styling-question.26451/