First of all, thanks for this lib, I'm amazed by how much it can do while being less than 1kb.
I have reduced my bundle size by a lot using this with preact.

Unfortunately I have run into a bug while extending styles.
Here is a minimal code sample to reproduce the bug:

import React from 'react'
import { render } from 'react-dom'
import { styled, setup } from "goober"

setup(React.createElement)

const RedBase = styled('div')`color: red;`
const Orange = styled(RedBase)`color: orange;`
const Blue = styled(RedBase)`color: blue;`
const UnderlinedBlue = styled(Blue)`text-decoration: underline;`

const App = () => {
	return <div>
		<Orange>Orange</Orange>
		<UnderlinedBlue>UnderlinedBlue</UnderlinedBlue>
	</div>
};

render(<App />, document.getElementById('app'))

In this situation the text in UnderlinedBlue should be blue, but it's red because of a wrong order of the class names in the class attribute. In that case, the classname of RedBase is after the classname of Blue and overwrite it.
Sorry the class names are actually in the right order. The problem most likely comes from the order in which the classes are declared in the generated CSS.

However if I remove <Orange>Orange</Orange>, it is blue as expected.

I am using goober 2.0.19-next.1
I will try to find some time to fix it.

happy new year !