<picture>
	<source media="(prefers-color-scheme: light)"  srcset="https://placehold.it/180/ffff00/000000?text=Light%20image">
    <source media="(prefers-color-scheme: dark)" srcset="https://placehold.it/180/002244/ffffff?text=Dark%20image">
    <img src="https://placehold.it/180/ffff00/000000?text=Light%20image%20fallback" alt="" />
</picture>

<div>
	<h1>Switch theme:</h1>
	<button onclick="setPicturesThemed(null)">Default (system)</button>
	<button onclick="setPicturesThemed('light')">Light</button>
	<button onclick="setPicturesThemed('dark')">Dark</button>
</div>
/***************************************

Presentational styles for demo only!

***************************************/

body {
	display: flex;
	font-family: sans-serif;
}

img {
	display: block;
	width: 100%;
}

div {
	display: flex;
	flex-direction: column;
	margin-left: 15px;
}

h1 {
	margin: 0;
	font-size: 1em;
	font-weight: normal;
	margin-bottom: 5px;
}

button {
	margin-bottom: 5px;
	white-space: nowrap;
}
function setPicturesThemed(colorScheme = undefined) {
	// Clean up all existing picture sources that were cloned
	document.querySelectorAll('picture > source[data-cloned-theme]').forEach(el => {
		el.remove();
	});

	if (colorScheme) {
		// Find all picture sources with the desired colour scheme
		document.querySelectorAll(`picture > source[media*="(prefers-color-scheme: ${colorScheme})"]`).forEach(el => {
			// 1. Clone the given <source>
			// 2. Remove the media attribute so the new <source> is unconditional
			// 3. Add a "data-cloned-theme" attribute to it for future reference / removal
			// 4. Prepend the new <source> to the parent <picture> so it takes precedence
			const cloned = el.cloneNode();
			cloned.removeAttribute('media');
			cloned.setAttribute('data-cloned-theme', colorScheme);
			el.parentNode.prepend(cloned);
		});
	}
}

External CSS

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External JavaScript

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