Page Template specific editor styles for the WordPress Block Editor (Gutenberg)

template-specific-editor-styles.js

const {
	subscribe,
	select
} = wp.data;

/**
 * Add dark-mode class to block-editor based on selected page template
 */
wp.domReady( () => {
	const postType = select( 'core/editor' ).getCurrentPostType();

	if ( postType === 'page' ) {

	    // get element to attach the darkmode class
	    var element = document.getElementById("editor");

		// Subscribe to editor state changes.
		let checkRequiredField = subscribe( () => {

			const template = select('core/editor').getEditedPostAttribute( 'template' ) // get selected page template

			if ( template.includes('dark') ) {
				element.classList.add("dark-mode-editor");
				element.classList.add("page-template-dark");
			} else {
				element.classList.remove("dark-mode-editor");
				element.classList.remove("page-template-dark");
			}
		} );
	}
} );