Bootstrap css not getting applied to the custom WordPress Dashboard Widget

Well, the add_action() should be

add_action( string $tag, callable $function_to_add, int $priority = 10, 
            int $accepted_args = 1 ) 

You’ve got the tag and then you have an array. That array should just be a string of ‘admin_theme_style’. That should call the entire function.

add_action()

Simply try:

add_action('admin_enqueue_scripts', 'admin_theme_style');