How to make a custom banner :3

Haiii, first blog am I right? :3 anyways.. Need help making a banner for your acc? just copy and paste this code into the "about me" section!!

<style>

main:before {

width: 100%;height: 300px;

display: block;

content: "";

background-image: url('ADD IMAGE URL HERE');

background-position: center;

background-size: cover;

}

</style>

Add ur desired image/gif into the spot for urls and u get ur own custom banner =3