On a side note, I hope and recommend they move to Stoat instead.

Tja

AFAIK there are only like 4 search engines in the world:

1. Google
2. Bing
3. Yahoo
4. Yandex

Virtually every search site uses one or a combination of these.

To generate electricity

I know you specifically requested a free service but if you can spare 1€ a month for it, I highly recommend mailbox.org

I don’t know about a non-javascript toggle but you can automatically apply dark theme styles with the “prefers-color-theme: dark” media query:

.theme-a {
  background: #ddccaa;
  color: #773311;
}
@media (prefers-color-scheme: dark) {
  .theme-a.adaptive {
    background: #775533;
    color: #ddccbb;
    outline: 5px dashed black;
  }
}

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules/@media/prefers-color-scheme

Sorry to be that guy but what is your question?