Add custom property for mono font stack

Closes #256

Author: pichfl

addon/styles/global.css

@@ -4,6 +4,8 @@
   --font-family-sans: "Inter var", "Inter web", -apple-system,
     BlinkMacSystemFont, Segoe UI, Helvetica, Arial, sans-serif,
     Apple Color Emoji, Segoe UI Emoji;
+  --font-family-mono: SFMono-Regular, Consolas, Liberation Mono, Menlo,
+    monospace;
 
   --color-white: #fff;
   --color-gray-100: #f4f6f8;
@@ -175,6 +177,11 @@ figcaption {
   color: var(--color-gray-600);
 }
 
+pre,
+code {
+  font-family: var(--font-family-mono);
+}
+
 /*
 @media (max-width: 1280px) and (min-width: 421px) {
   body,