_is-color validator fails on CSS Custom Properties

[GoOz]

Description

I would like to use the triangle() mixin with a CSS Custom Property as the color parameter but it obviously fails because the _is-color validator doesn't recognize the custom property as a color, which seems legit, but I think it should accept in some way CSS Custom Properties as they are more and more used, specially within a themable context.

Steps to Reproduce

.element {
  @include triangle($arrow-direction, $arrow-width, $arrow-height, var(--my-triangle-color));
}

• Bourbon version: 7
• Platform: any