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.