[Steven Zahm]

@ Don

I just installed Twenty Seventeen and I was able to use the Customizer to change the theme settings without issue.

My suggestion is to try to increase the memory allocated to PHP. Customizer required quite a bit a memory so perhaps you are exhausting the memory which has been allocated to PHP.

Here’s an excellent blog post which covers several methods. I suggest starting at the top and working your way down the list. The method that will work depends entirely on the web host. Each host is configured differently so one method may not work but another will.

http://dailyblogging.org/wordpress/increase-wordpress-memory-limit/

Hope this helps! Let me know.