Polymer 2 styling an element's child node from an outside stylesheet - css

Question

Let's say that I have a custom web element called <my-course> with its own style defined in the <style> tag inside the definition and I do not want to alter this element's file at all as it's an external dependency of my project.
This <my-course> element has a <div> child defined in the <template> tag.
Example:
<dom-module id="my-course">
<template>
<style>
::host {
padding: 5px;
background: rgba(0,0,0,.2);
}
div#progress {
height: 20px;
width: 100%;
background: red;
}
</style>
<h1>This is my custom course element</h1>
<div id="progress"></div>
</template>
<script>
class MyCourse extends Polymer.Element {
static get is() {
return 'my-course';
}
}
window.customElements.define(MyCourse.is, MyCourse);
</script>
</dom-module>
I want to make the div#progress green with "background: green;" (it's red by default) via an external stylesheet that is loaded in the same page as the custom element is attached/used.
I tried to do:
my-course div#progress {
background: green;
}
But it does not work, the progress div keeps being red. There seems there is no way to style the shadow dom from outside the element itself, I've tried my-course::content div#progress, and has no result (/deep/ and ::shadow are deprecated) I previously achieved this using ::shadow.
Anyone can help? Thanks

Answer 1

You should use CSS variables, such as:
::host {
--progress-background: red;
padding: 5px;
background: rgba(0,0,0,.2);
}
div#progress {
height: 20px;
width: 100%;
background: var(--progress-background);
}
And to overrride it:
my-course {
--progress-background: green;
}
More info here: https://www.polymer-project.org/2.0/start/first-element/step-5