Override fomantic monospace fonts and set size (#13435)

We did not override those yet so default `monospace` was applied. Also
added a tweak to slightly reduce font size because monospace fonts
usually render slightly larger than proportional ones.

Co-authored-by: Antoine GIRARD <sapk@users.noreply.github.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_base.less

@@ -123,6 +123,13 @@ h5,
   font-family: var(--fonts-regular);
 }
 
+code,
+kbd,
+samp {
+  font-size: .9em; /* compensate for monospace fonts being usually slighty larger */
+  font-family: var(--fonts-monospace);
+}
+
 body {
   background-color: var(--color-body);
   overflow-y: auto;