`$button-color-alt` is unused/ignored

[joekrill]

How can we reproduce this bug?

There's a variable called $button-color-alt that is documented as:

/// Text color of buttons with a light background.
/// @type Color
$button-color-alt: $medium-gray !default;

But that variable is never used or referenced anywhere else in the codebase. It looks like it had been previously used for secondary buttons, but was removed in this commit which is just described as "updating button tests". That was in 2016.

What did you expect to happen?

Based on the variable name, I would expect that $button-color-alt would be used for buttons with a light background. Or, at the very least, the variable should probably just be removed to avoid confusion. But it would be nice to allow this in order to be able to handle cases where someone changes one of the color variables to a lighter shade.