You can modify or edit any theme by going to your account/My Themes, find the theme you want to change and click on Edit. You can dowload a new/edited header or footer and even change the colr schemes you chose.
Theme headers have to be 3000 x 200 pixels for different screen sizes. The “main” part of the header should be in the far right side of your screen. Some users like a design covering all 3000 and some don’t so you try to fade into the left side.
I hope this helps you.