Unable to adjust the width

Hi,

First of all, this is a great and simple plugin. Thanks.

I’m having trouble adjusting the width. This is how it looks like on my blog.

http://awesomescreenshot.com/0292qauq0d

I would like to be able to adjust the background color blue, in this case, to match the width of my blog. Right now, it’s all full width. How do I do that?

Thanks.

https://ww.wp.xz.cn/plugins/sticky-header/