In +page.svelte
<script>
import ThemeToggle from '<PATH>/ThemeToggle.svelte';
</script>
<ThemeToggle />
On mount, the button will be unhidden and get the current theme from either (in the following order):
If the user does not have the localStorage key, it will be set to the selected theme.
The theme will also be set as a data attribute on the HTML element so you can use that value (light or dark) when setting your styles.