On some occasions, a Font will be unresponsive in the editor, perhaps not changing as you change fonts in the Theme section of the editor or not sizing properly. When this happens, typically the problem is one of two things:
1. You need to re-boot your computer if you have been editing a lot to clear out your cache.
2. You have cut and pasted from a word processor and some code from the word processor has been carried over into the Site Editor. Even though you may not see the word processor code it is there and can cause problems with font and other display attributes.
When cutting and pasting, always use a plain text editor, not something that formats the text.