CSS parser doesn't handle colors in functions

For example this isn't converted as it should:

```css
color: linear-gradient(rgba(15, 20, 25, 1), rgba(15, 20, 25, 0));
```

It comes from https://github.com/rust-lang/rust/blob/master/tests/rustdoc-gui/scrape-examples-color.goml.