docs hightlight fixes, dark mode improvements

Gemfile.lock

@@ -3,6 +3,7 @@ GEM
   specs:
     addressable (2.8.1)
       public_suffix (>= 2.0.2, < 6.0)
+    coderay (1.1.3)
     colorator (1.1.0)
     concurrent-ruby (1.1.10)
     em-websocket (0.5.3)
@@ -49,6 +50,9 @@ GEM
       rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
+    kramdown-syntax-coderay (1.0.1)
+      coderay (~> 1.1)
+      kramdown (~> 2.0)
     liquid (4.0.3)
     listen (3.7.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
@@ -79,6 +83,7 @@ DEPENDENCIES
   jekyll-redirect-from
   jekyll-tidy
   jekyll-timeago
+  kramdown-syntax-coderay (~> 1.0)
 
 BUNDLED WITH
    2.1.4