Hi there,

Thanks for reaching out.

The “Error: undefined” message when adding Custom CSS is not expected behavior and is usually not due to a specific limitation on CSS itself.

This can happen due to:

• A syntax issue in the CSS (missing brackets, semicolons, etc.)
• Invalid or unsupported CSS being parsed by the editor
• A JavaScript conflict in the backend
• Cache-related issues

Please try the following:

• Double-check your CSS for any syntax errors
• Add the CSS in smaller parts to identify the problematic line
• Clear Joomla and browser cache
• Try using a different browser or incognito mode
• Check the browser console for any JS errors

If the issue persists, please share the exact CSS snippet you are trying to add so we can review it more closely.

Best regards,