Issue with CSS :root definitions

Hi,

Using the https://validator.w3.org <https://validator.w3.org/> for validation of my PHP-HTML-CSS very simple webpage https://www.harmfrielink.nl/Users/Error404.php
 <https://www.harmfrielink.nl/Users/Error404.php>I have the following coding-snippet that causing issues:

<style>
:root {
   --404-border      : 1px solid white;
   --404-border-blue : 1px solid blue;
   --404-text-color  : #FFFFFF;
}
</style>



Errors (8) · Hide all errors <https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.harmfrielink.nl%2FUsers%2FError404.php> · Show all errors <https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.harmfrielink.nl%2FUsers%2FError404.php>
CSS: ___: Parse Error. (8) · Hide all <https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.harmfrielink.nl%2FUsers%2FError404.php> · Show all <https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.harmfrielink.nl%2FUsers%2FError404.php>
CSS: --: Parse Error. (3)
CSS: border: Parse Error. (3)
CSS: color: Parse Error. (2)

For me this code is just-as-it-has-to-be.
The code is working like charm!

What's suggestion is the improvement?

regards / met vriendelijke groet / mit freundlichen Grüßen,
HJM (Harm) Frielink
Schlehenweg 21
D-49828  Neuenhaus (Germany)

email:      harm@harmfrielink.nl
home:     +49 5941 20 59 592
mobile:     +31 6512 939 77
handy:     +49 178 63 20 776
www:      https://www.harmfrielink.nl

p.s.:      NSA please read the constitution, not my mail.