Replace dark stylesheet with media query

We have media queries now so we can replace all the stylesheet loading and unloading etc with:

```
@media (prefers-color-scheme: dark) {
}
```

Search the repository for the existing dark stylesheet loading and unloading code, then inspect how the library currently applies dark styling. Replace the stylesheet lifecycle with the media-query approach and verify that dark-mode styling still changes correctly without dynamic loading or unloading.

Written by the indexing model from the issue text.