I’ve published a UserStyle at userstyles.org that anyone can install (provided their browser has a compatible styling extension installed) to change the default fixed-width font on the site.
The built-in (and apparently un-modifiable) Discourse default, Consolas, has poor differentiation between some characters. (Most notably, the lowercase-l and the numeral 1.) That makes it a poor font for displaying code snippets and such, which is a pretty critical need for this site.
By default, the userstyle will make Liberation Mono the default font, but it also gives you the option to select Lucida Console, Menlo, or Monaco, if you prefer those options. (These are all fonts supported by the original Discourse CSS, the userstyle simply shuffles around what order they’re listed to change the priorities.)
It makes the difference between 1l1l1l1l1l1l1l1l1111llll much more obvious than it is when using the default Consolas font.
You can find it here: https://userstyles.org/styles/174558/ask-fedora-cleaner-fixed-width-fonts