Add dark mode (#199)

* Add passive dark mode to everything but codemirror

* Add dark theme support for Codemirror

* Make theme a user setting

* Fix button text size

* guard against undefined window

* Formatting and test fix

tailwind.config.js

@@ -38,5 +38,6 @@ module.exports = {
       },
     },
   },
+  darkMode: 'class',
   plugins: [],
 }