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' );

function jett_block_styles() {
	register_block_style(
		'core/heading',
		array(
			'name'         => 'gradient',
			'label'        => __( 'Gradient', 'jett' ),
			'inline_style' => '
				.wp-block-heading.is-style-gradient {
					background: var(--wp--preset--gradient--violet-to-green);
					-webkit-background-clip: text;
					-webkit-text-fill-color: transparent;
					line-height: 1.1;
				}'
		)
	);
	register_block_style(
		'core/paragraph',
		array(
			'name'         => 'gradient',
			'label'        => __( 'Gradient', 'jett' ),
			'inline_style' => '
				p.is-style-gradient {
					background: var(--wp--preset--gradient--violet-to-green);
					-webkit-background-clip: text;
					-webkit-text-fill-color: transparent;
					line-height: 1.1;
				}'
		)
	);
}
add_action( 'init', 'jett_block_styles' );