svelte/documentation/docs/04-styling/03-custom-properties.md

title
Custom properties

You can pass CSS custom properties — both static and dynamic — to components:

<Slider
	bind:value
	min={0}
	max={100}
	--track-color="black"
	--thumb-color="rgb({r} {g} {b})"
/>

The above code essentially desugars to this:

<svelte-css-wrapper style="display: contents; --track-color: black; --thumb-color: rgb({r} {g} {b})">
	<Slider
		bind:value
		min={0}
		max={100}
	/>
</svelte-css-wrapper>

For an SVG element, it would use <g> instead:

<g style="--track-color: black; --thumb-color: rgb({r} {g} {b})">
	<Slider
		bind:value
		min={0}
		max={100}
	/>
</g>

Inside the component, we can read these custom properties (and provide fallback values) using var(...):

<style>
	.track {
		background: var(--track-color, #aaa);
	}

	.thumb {
		background: var(--thumb-color, blue);
	}
</style>

You don't have to specify the values directly on the component; as long as the custom properties are defined on a parent element, the component can use them. It's common to define custom properties on the :root element in a global stylesheet so that they apply to your entire application.

[!NOTE] While the extra element will not affect layout, it will affect any CSS selectors that (for example) use the > combinator to target an element directly inside the component's container.