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

src/routes/Onboarding/Camera.tsx

@@ -10,10 +10,10 @@ const Units = () => {
 
   return (
     <div
-      className="fixed grid justify-center items-end inset-0 bg-chalkboard-110/50 z-50"
+      className="fixed grid justify-center items-end inset-0 bg-chalkboard-110/50 dark:bg-chalkboard-110/80 z-50"
       style={{ clipPath }}
     >
-      <div className="max-w-2xl flex flex-col justify-center bg-white p-8 rounded">
+      <div className="max-w-2xl flex flex-col justify-center bg-chalkboard-10 dark:bg-chalkboard-90 p-8 rounded">
         <h1 className="text-2xl font-bold">Camera</h1>
         <p className="mt-6">
           Moving the camera is easy. Just click and drag anywhere in the scene