How to write code to the background
It's all very strange, try to reformulate.
Describe what is not working and attach a picture should look like.
Your code gave jsfiddle.net or counterparts.
Is there a mistake?
Use normal editors and they'll all highlight.
Are you sure the div is a class? And why is it 3 times, I think the browser the first time is poorly understood?
2. Between the numbers and px do not write a space