Made code colors dark in MD theme

boards/static/css/md/base_page.css

@@ -416,7 +416,15 b' ul {'
     padding-left: 0px;
 }
 
+/* Reflink preview */
 .post_preview {
     border-left: 1px solid #777;
     border-right: 1px solid #777;
 }
+
+/* Code highlighter */
+.hljs {
+    color: #fff;
+    background: #000;
+    display: inline-block;
+}