Background transparency issue with iframe elements

Hi there, and thanks for pico.css, first of all.
I could not pinpoint how or why yet, but it looks like pico is somehow messing up with iframe elements transparency. I noticed it when trying to add a widget to my website, but was able to reproduce with a number of other ones.
Whatever the iframe content, it always ends up being displayed with a white background, which clashes with the main page.

I've added a minimal example to reproduce the issue (to be used with the system/browser set to dark mode).

First one, without pico, where the iframe does correctly pick up the background color:
```

```

Second one, with pico, where the iframe always displays a white background:
```

```

I can't find anything that would set that directly, but it looks like it is tied to the color-scheme property:
- https://www.reddit.com/r/css/comments/14axek9/why_does_iframe_have_white_background_when_inside/
- https://fvsch.com/transparent-iframes

Now, not sure if there's anything that can be done in pico ?