Hello @jnlinn,
Actually, you can ignore those errors because those are CSS Variables which is a new CSS technology that the W3 Validator is not yet supporting. Be advised that the W3 Valitor only checks the HTML structure and not the CSS. It says something like this:
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available. As an alternative you can also try our non-DTD-based validator.
In terms of new technologies, the checker may not have applied it yet as the website itself also says that it is an experimental.
Best Regards.