Steps to replicate the issue (include links if applicable):

Edit a template's stylesheet, e.g. https://en.wikipedia.org/wiki/Template:Example/sandbox/styles.css to include the following:

.whateverclass {
	border-color: var(--border-color-content-added, #C7D0F8);
}

What happens?:

Saving is refused with an erroneous validation message: “Invalid or unsupported value for property border-color at line 2 character 16.”

What should have happened instead?:

This code is valid and should have been accepted.

Note that, if you change ‘border-color’ to ‘color’ or ‘border’, it is accepted just fine, as it should be.

Other information (browser name/version, screenshots, etc.):

Firefox 145.0.2 on macOS 15.7.1.