Threads and Folder Pane fonts and backgrounds can be changed with css.

/* threads pane bg, font */
table[is="tree-view-table"]{
background: #87CEFA !important;
font-family: Times, serif !important;
font-size: 16px !important;
color: navy !important;
}

/* folder pane font, bg */
#folderPane {
background-color: #87CEFA !important;
font-size: 14px !important;
font-family: Times, serif !important;
color: navy !important;
}

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the numbers and colours as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

Thanks for this very extensive recipe, it works!

Yes it works thanks! Shouldn't be so tricky to change a font but there you go.

This is all very cool, but it begs the question, "what else can I do?"

I see these 2 things, the folder pane, and the threads pane (although it seems to be called something different.)

I also ran across the toolbar-menubar in another post on this site.

It occurs to me there might be a lot more that can be done here.

Is there any good documentation on everything I can change?

Thanks, Mark.

This is all very cool, but it begs the question, "what else can I do?" I see these 2 things, the folder pane, and the threads pane (although it seems to be called something different.) I also ran across the toolbar-menubar in another post on this site. It occurs to me there might be a lot more that can be done here. Is there any good documentation on everything I can change? Thanks, Mark.

Just about every UI element can be modified with css, but the documentation is scattered among different forums, including this one, mozillazine, elevenforum, reddit etc.