blocks customize

Hey, was trying to add a site icon (aka favicon) through Appearance->Customize->Site Identity but kept getting unspecified error message. Disabled iThemes Security and was able to add it no problem.
I know your default installation settings disable the Editor and it took me a while to find the proper setting to undo that. I am wondering if there is another setting I have to search forever for in order to allow me to use the Customize feature properly? Or is that just blocked regardless of settings?
Thanks!