Demo app to show how themes might work

Important files:

• src/routes/layout.svelte: sets theme class on document root element. Not the cleanest thing, but svelte doesn't support anything better right now (see this GH issue). It's pulling from the store, because I imagine that when this is actually implemented, we will want to decide the theme based on information (user grade level) from the store.
• themes.scss defines the custom css variables for the themes.

Testing it out

npm install

And then:

npm run build
npm run preview

For some reason, npm run dev doesn't get the styles showing up correctly.