Hi @nikkers and @zecompadre,
thank you for reporting the issue. We just released the new version of this plugin, where CSSTidy library is entirely removed – please update the plugin to version 2.4.0 to get rid of these PHP warnings.
Details: instead of CSSTidy library, we used the same CSS sanitization approach as WordPress core uses for CSS defined in customizer or theme.json – that means, it’s up to the user to provide valid CSS, and it’s up to us to ensure its stored and rendered securely. With this context, we decided that CSSTidy library is a bit too heavy for this simple use case, hence the removal.
Hope that makes sense!