You can extend the existing theme by writing a new one.
From this blog post I'm going to show how to change styles (background colours, fonts etc.)
First, Create a directory called "carbon.super/themes" inside <EMM_HOME>/repository/deployment/server/jaggeryapps/store/themes/.
Then, Create a directory called "css" inside <EMM_HOME>/repository/deployment/server/jaggeryapps/store/themes/carbon.super/themes.
Add below two css files to above created css directory. You can change it's value based on your preferences.