Added Plex mono

Author: cxharris

docs/_includes/head.html

@@ -19,6 +19,7 @@
 <!-- Added by C Harris on 5th June 2019 -->
 <style>
     @import url('https://fonts.googleapis.com/css?family=IBM+Plex+Sans&display=swap');
+    @import url('https://fonts.googleapis.com/css?family=IBM+Plex+Mono&display=swap');
 </style>
 
 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script>

docs/css/customstyles.css

@@ -3,6 +3,14 @@ body {
     font-size:15px;
 }
 
+pre {
+    font-family: 'IBM Plex Mono', 'Courier New', Courier, monospace;
+}
+
+code {
+    font-family: 'IBM Plex Mono', 'Courier New', Courier, monospace;
+}
+
 .bs-callout {
     padding: 20px;
     margin: 20px 0;
@@ -821,7 +829,7 @@ a.accordion-toggle {
 
 span.red {
     color: red;
-    font-family: Monaco, Menlo, Consolas, "Courier New", monospace;
+    font-family: 'IBM Plex Mono', Monaco, Menlo, Consolas, "Courier New", monospace;
 }
 
 h3.codeExplanation {