To begin with, my best advice for getting css styles to work as coded is to read books as standard epub, not kepub. If you're a calibre user it would require using the standard KOBOTOUCH driver not the Kobo Touch Extended driver plugin.
However, to get monospace fonts requires some more work. Kobos don't have a built-in monospace font (odd, I know). You can try embedding the 4 monospace files (regular, italic, bold, bolditalic) into each and every book if you like but I prefer to sideload them once to the Kobo /fonts directory and then externally reference them from the epub.
Read this post for more details about how to code the relevant css @font-face statements. I use HP's Dark Courier font, which is free.
If you haven't sideloaded fonts before post #1 of
this thread should help.
If you're a calibre user and you don't want to hand-edit every epub, take a look at the
Modify CSS option in the KOBOTOUCH driver. Adding the monospace @font-faces to the kobo_extra.css file on your Kobo will mean that they get auto-added to every epub during the calibre send-to-device process. This is the method I've been using for ages, it's painless and it works.
If the above isn't detailed enough feel free to ask and I can elaborate where necessary.