All you need for that is a user stylesheet with something like the following:
#mainbar code {
font-family: Serif !important;
}
Serif might not be the best selection here :). The #mainbar
selector should make it match all over SE, and hopefully not too much elsewhere.
For Chrome, you need to put that in your Default/User Stylesheets/Custom.css
file. (Not sure where that lives on Windows. On Linux, should be under ~/.config/chrome
.)
Given that most modern browsers let you have user stylesheets, and that extensions for doing things like this exist (like http://userstyles.org for instance, with Stylish for Chrome, plugin also exists for Firefox), I don't think there is enough justification to having per-user settings for this.
If you give the option just for the code-block font, someone's going to ask for some other font tunable, or color, or spacing, or... and that's probably going to end up being a QA nightmare.
font-family: Consolas, Menlo, Monaco, Lucida Console, Liberation Mono, DejaVu Sans Mono, Bitstream Vera Sans Mono, Courier New, monospace, serif
. Not your first choice indeed, so the user stylesheet is a better fix. But other sites will probably have similar fallbacks defined.