style(themes): Add GitHub dark theme (anuraghazra#758)

This styling falls in line with the recently released GitHub dark mode

Co-authored-by: Anurag Hazra <[email protected]>

themes/index.js

@@ -261,7 +261,7 @@ const themes = {
     title_color: "bf4a3f",
     icon_color: "a64833",
     text_color: "d9c8a9",
-    bg_color: "402b23",
+    bg_color: "402b23"
   },
   outrun:{
     title_color: "ffcc00",
@@ -281,6 +281,12 @@ const themes = {
     text_color: "718CA1",
     bg_color: "1D252C",
   },
+  github_dark: {
+    title_color: "58A6FF",
+    icon_color: "1F6FEB",
+    text_color: "C3D1D9",
+    bg_color: "0D1117"
+  }
 };
 
 module.exports = themes;