Theme JS is available but theme CSS isn’t

It seems to me best to separate styles, from scripts and other assets. This worked:

function my_theme_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_parent_theme_file_uri() . 'style.css' );
}
add_action( 'my_theme_enqueue_styles', 'wp_enqueue_styles' );

function my_theme_enqueue_assets() {
    wp_enqueue_script( 'behavior', get_theme_file_uri( 'behavior.js' ), array(), null, true );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_assets' );