Can’t change capabilities

Hello Alex,

I’ve tried your snippet to change capabilities for the tools menu:

add_filter(
	'regenerate_thumbs_cap', function() {
		// See https://codex.ww.wp.xz.cn/Roles_and_Capabilities
		return 'upload_files';
	}
);

The snippets works perfectly, if I add it directly to the regenerate_thumbnails.php, but refuses to work when added to the functions.php of a theme. I also tried to wrap it in a function rt_capabilities() and add_action( 'after_setup_theme', 'rt_capabilities' );, but to no avail.

What do I miss?