Create a file in the template directory named style.css and put your css in there; in setting the new font, mark it !important:
div.dokuwiki code {
font-family: my_font !important;
}
See design.css, where the code font is defined:
div.dokuwiki code {
font-family: monospace, serif;
/* second generic font fixes problem with font-size, see
http://meyerweb.com/eric/thoughts/2010/02/12/fixed-monospace-sizing/ */
font-size: 100%;
}