Doesn't accept CSS variables

[hugopeek]

Describe the bug

The linter doesn't seem to accept CSS variables.

To Reproduce

Introduce a CSS variable:

:root {
  --border-radius: 3px;
}

This throws an error:

Unknown CSS property "--border-radius" (line: 1980, char: 20)

Expected behavior

CSS variables are valid CSS.

https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties