jett-theme/functions.php

<?php

/* Remove default CSS variables that come with Wordpress
  https://github.com/WordPress/gutenberg/issues/56180#issuecomment-1819222376
*/
function custom_wp_theme_json_default( $theme_json ) {
	$new_data = array(
		'version'  => 2,
		'settings' => array(
			'color'  => array(
				'palette'   => array(),
				'gradients' => array(),
			),
			'shadow' => array(
				'presets' => array(),
			),
			'typography' => array(
				'fontSizes' => array(),
			),
			'dimensions' => array(
				'aspectRatios' => array(),
			),
			'spacing' => array(
				'spacingScale' => array(
					'steps' => 0,
				),
			),
		),
	);
	return $theme_json->update_with( $new_data );
}
add_filter( 'wp_theme_json_data_default', 'custom_wp_theme_json_default' );