re : I tried the Theme and Font Changer add-on but I can't get it to install.

https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer/

click on 'download now' button it will ask if you want to open or save file. select 'Save file' click on OK

The file will download and be saved in the default folder specified on your computer for files you download. In my case it is the 'Downloads' folder in my 'User Account'.

In Thunderbird 'Menu icon' > 'Add-ons'

a new tab opens showing your addons.

click on the gear wheel icon and select 'Install Addon from file' see image below

locate and select the 'theme_font_size_changer-55.0-fx+sm+tb.xpi' file you downloaded and click on 'Open'.

The addon will be installed. You may need to restart thunderbird at the prompt.

I don't see any gear-whell icon in the add-ons page. Pic attached.

click the green puzzle piece, than you will see what toad-hall speaks of.

Ohh and click the keep button so the install and update can actually complete for the calendar. Leaving it in limbo like that might cause other issues down the track.

You have a very minimalist style there. I'd ordinarily say "click on the extensions tab" but as you don't have any text labels, that wouldn't help you. Click on the next tab down, with the green jigsaw piece.

Some of the things the developers do are insane. The natural place for a feature that helps with downloaded addon files would be the tab that talks about downloading addons. But no, it's on all the other tabs.