First, take a close look at the following code snippet:

function lmscoder_purplified_focus_mode() {
	$script = "
		const style = document.createElement( 'style' );
		const css = `
			* {
					color: purple !important;
					background-color: purple !important;
					border-color: purple !important;
					outline-color: purple !important;
			}
		`;
		
		style.appendChild( document.createTextNode( css ) );

		document.getElementsByTagName( 'head' )[0].appendChild( style );
	";
	
	echo '<script>' . $script . '</script>';
}
add_action( 'learndash-focus-template-end', 'lmscoder_purplified_focus_mode' );