If a rule does not work when placed in the theme global custom css rule box, check two things.
first try adding !important to the rule
h1 {font-size: 36px !important;}
And if it still does not work review the whole content of the CSS rule box in search for syntax errors that would break the file
You can c0py paste the content on the custom css rule box in an online CSS validator for that.
If you still can’t figure it out. Leave the rule and post back with a link to a page where its effect should be visible, and precise what element in the page is supposed to be affected