One of the statements you refer to does go in config. Do that. For the others, you need to go to your profile and create a folder called 'chrome' (lower case). In that folder, create a simple text file called userChrome.css (case sensitive). In that folder, store the CSS statements from the link you referenced. This takes effect the next time you start Thunderbird.

Update: also include this statement at top of the userChrome.css file:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

What's the namespace url for? It's working without.

Thanks! That was easy enough! Much better that everything isn't in the nearly same pale yellows and blues.

Now I'll know how to add css the next time I want to, as well.

That comment used to be required. If you're pleased, I ask you to mark this solved, so it will be removed from the list of open issues. Thank you.

I don't think you can comment once it's solved, so I left it open for a reply. Thanks! I put it in comments in the file in case it turns out I need it, later.

THANK YOU! This has been frustrating me for a while!

I followed all the advice I could find, tested and proved my userChrome.css file was functioning, but the script suggested no longer appears to resolve this for me (since a laptop rebuilld). (I used the script below and set and set my toolkit.legacyUserProfileCustomizations.stylesheets preference to "True".)

.calendar-month-day-box-current-month[selected="true"] {

 background-color: orange !important;

}

.calendar-month-day-box-current-month[relation="today"] {

 background-color: red !important;

}