Themes as such are defunct starting with V68. There is no what to change the things themes used to change.

There are a couple of Themes that have been converted to addons to get around the change. Have a look here. https://addons.thunderbird.net/en-US/thunderbird/search/?q=theme&appver=68.7&platform=

So far, no addons (themes) that control the fonts and presentations on the screen of all words and letters. Thanks anyway. GWM

The fonts for the Threads and Folder Panes, the menus and other interface elements can be set with css files. There are several examples in these topics:

https://support.mozilla.org/en-US/questions/1269361

https://support.mozilla.org/en-US/questions/1271886

The fonts for message content are usually set as follows: Tools/Options/Display/Formatting, Advanced... set the Sizes and fonts with Fonts for: Latin and repeat with Fonts for: Other Writing Systems, uncheck 'Allow messages to use other fonts'.

sfhowes - your solution to my problem was spot on. I followed your instructions and when I first modified the css numbers to 1.25, all fonts went very small. I right clicked again, and put in 2.5 and all went larger than I wanted, so I went smaller in increments and finally everything settled at 1.20 and is very nice and readable now. Thank you very much. GMoss

sfhowes - The solution to my original problem was taken care of, and that is the size of the font that I adjusted with your kind assistance. The problem I am still having is that the font for the "read" emails, is fine, and very hard to read as it has no body to the type. How can I get the font to be more substantial, and darker for the already read headers and bodys????