I am having this issue too. No icons in the top right. I'm not able to find all the fonts under the Design> Site Styles option! I made sure by changing every single font option to the same exact one, but my Headers etc are uneditable.
I've included some screenshots. Also I have edited this site before with the icons on the top right. I did recently change the theme, but I had never heard of a theme where some of the fonts are uneditable.