Hi Michael,
It seems that you may have customized the theme main style.css file directly rather than using custom css options. You can add your Custom CSS (styles) in Appearance -> Customize -> Additional CSS and paste your custom css code.
So that if you activate the pro version your customized code will not lost. We always recommended not to customize the code or if you customize the code then while updating to new version or upgrading to pro version all your customized code will be lost. So better create a child theme if code customization is required.
Also the theme CSS validation code error that you have provide above is not actual an error since the validator system is validating with the same old system and need to update to a latest version of CSS.
Thank you!