Were you using the Theme Font & Size Changer add-on? It stopped working yesterday. Here is a revised version:

http://www.rossde.com/ThemeFontSizeChangerNoexpire/index.html

…and here is guidance on how to install from an xpi file:

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Thanks for your reply. Theme Font and Size Changer only works in the Menu Bar area. I need to change font's in the Message Reading area, thus Tools>Options>Display>Formatting.

If you look at the last image in this website:

http://xenos-email-notes.simplesite.com/417754775

...have you set the sizes for "Latin"?

Yes I have tried that. It is set to Latin. It doesn't matter what Size or Color it is set to, there is no response. I believe this problem began with the new version, 52.5.2 download. I also installed Thunderbird on another Win 10 PC today and have the same problem. This download version is 52.5.2, the current version. As I indicated earlier, I reinstalled the program and have the same results. Thanks for your input.

Strange, that is still working here in Windows 10.

OK. Try changing it from "Latin" to "Other writing systems" and set the sizes there too.. That seems to be the option that relates to unicode/utf-8 fonts.

No, that didn't work.