svelte/documentation/tutorial/13-advanced-styling/03-inline-styles/app-b/App.svelte

<script>
	let bgOpacity = 0.5;
	$: color = bgOpacity < 0.6 ? '#000' : '#fff';
</script>

<input type="range" min="0" max="1" step="0.1" bind:value={bgOpacity} />

<p style="color: {color}; --opacity: {bgOpacity};">This is a paragraph.</p>

<style>
	p {
		font-family: 'Comic Sans MS', cursive;
		background: rgba(255, 62, 0, var(--opacity));
	}
</style>