Everything you’re able to do in a plugin you could do in a theme. It’s more like that separation is preferred, see article: https://managewp.com/blog/themes-design-plugins-functionality
Below an example adding editor stylesheet:
/**
* Registers an editor stylesheet for the theme.
*/
add_editor_style( array(
'assets/css/bootstrap.min.css',
'https://use.fontawesome.com/releases/v5.3.1/css/all.css',
'style.css',
));
Below an example to add scripts to the WP admin backend.
function admin_scripts() {
wp_enqueue_style( 'bootstrap', get_theme_file_uri( '/assets/css/bootstrap.min.css' ), array(), null );
wp_enqueue_script( 'bootstrap-bundle-js', get_theme_file_uri( '/assets/js/bootstrap.bundle.min.js' ), array('jquery'), null, true );
}
add_action( 'admin_enqueue_scripts', 'admin_scripts' );
If you need to load your scripts earlier for whatever reason, try admin_init
below
add_action( 'admin_init', 'admin_scripts' );