Loading CSS in a custom plugin

How should CSS be loaded correctly in a custom plugin?

I’ve a function onAssetsInitialized which is already loading JS successfully in the plugin.

And am trying to load like the below but this isn’t doing anything…

$assets->addCss('plugins://paypal-checkout/assets/paypal-checkout.css');

Ah never mind. This is actually now loading.

I don’t know why it wasn’t a minute ago.