Breaking change introduced in v11.1.0

A breaking change related to custom css was silently introduced in v11.1.0:
> CSS now injected at mount via useStyleSheet (prerequisite for nonce).

This causes a different injection order for css styles, causing the [override of css variables](https://fkhadra.github.io/react-toastify/how-to-style/#override-css-variables) not to work anymore unless the `!important` tag is added.