Wordpress CSS Help! How to make Header 100% same for every Monitor size?

Hey there guys I just built a site on Wordpress using the OptimizePress theme, but I can't seem to figure out how to get my Header Background to stay the exact same way for every different sized Monitor that it's viewed on.

I know you have to edit the CSS (To make the Header set to 100% width), but I'm still really new to CSS and am unsure what code will make this work, haha.

What is the exact code I need to use?

I've tried adding a few different code variations that haven't changed anything at all through Page Settings > Other Scripts > Custom CSS through the OptimizePress panel.


Any help would be great, thanks guys!

- I need to do a similar website pages. Who can help me?

What is website url? Or if you do not want to make it public send me an PM and i will try to help you.