randomize-background.php

<?php

/**
 * Randomize theme background colors
 */
add_filter(
	'wp_theme_json_data_theme',

	/**
	 * Filter the theme JSON data
	 *
	 * @param \WP_Theme_JSON_Data $theme_json The theme.json data class object to access and update the underlying data.
	 *
	 * @return \WP_Theme_JSON_Data Same instance as provided.
	 */
	function ( \WP_Theme_JSON_Data $theme_json ): \WP_Theme_JSON_Data {
		$colors = [
			'#8D6A9F',
			'#C5CBD3',
			'#8CBCB9',
			'#DDA448',
			'#BB342F',
		];

		return $theme_json->update_with(
			[
				'version' => 2,
				'styles'  => [
					'color' => [
						'background' => $colors[ wp_rand( 0, count( $colors ) - 1 ) ],
					],
				],
			],
		);
	}
);