Use color-yiq for outline button text.

[agriffis]

Replace a hard-coded #fff with color-yiq for text color.

[andresgalante]

Hi @agriffis thanks for the contribution. I don't think we need a default value for $color-hover in this case since we are defining for every hover color but it doesn't hurt to remove the hardcoded.

[agriffis]

Hi @andresgalante, thanks for the reply.

Regarding the default, I think customizers like myself would appreciate having it, otherwise I'll just be calling color-yiq on the way. It seems to make sense for button-outline-variant considering button-variant uses color-yiq for its text color. Do you disagree?

[andresgalante]

@agriffis you are right, it's good to have it to extend bootstrap

[mdo]

Resolved the conflict here and put the $color-hover variable to use in the mixin. Turns out we weren't even using it there; we were overriding that value with an existing color-yiq function.

[mdo]

Actually, to track that right, I cherry-picked your commit into #25339.