Change Theme or Customize Theme

I am using the Responsive Theme, 4.2.2 version of WP,and I made a child version of Responsive in case any updates to the theme might cause problems. (Or in case I made changes to the theme that I might later need to undo since I’m not sure what I’m doing.) I want to change the background color of my theme and possibly even change the theme itself, but when I select customize (or even background) while in the WP admin dashboard, the little circle comes up and rotates as though it’s loading, but it never loads. I let it spin for 30 minutes and it still never loaded. Suggestions please?