I adjusted it to Pico V2 in a Astro component, ready to use in a Astro project with Pico V2 as CSS framework.
Since the component is just one file i just a made a gist: https://gist.github.com/ivoba/f3e0a8d7423faf318ce2217f5af9c510

Here is how to use it, f.e. in your navigation:

---
import ThemeSwitcher from "./ThemeSwitcher.astro";
---

...
  
<li>
  <ThemeSwitcher />
</li>

Update:
I added a persistence layer for localStorage to the Gist.
So the selected theme is remembered between pages.