This isn't elegant at all, and is actually rather annoying for others, but it fixes a problem, so here it goes.
This PR aims to improve the color contrast on hover and active state for light and dark buttons. There's no other way I can find to do this without it being a complete rewrite of the component at this point. Placing two custom conditions in here though—based on the extremes of our color palette—seems okay though.
I'd rather go this route than #35293 because this keeps the customization in the usage of the mixin as opposed to the logic of the mixin. Ideally we could setup custom color functions like #34013, but I can only see us doing that with a rewrite of the mixin as well (which could include an optional parameter or something for specifying tint or shade.
Lastly, I don't see this needing to affect outline buttons. Those have separate issues, yes, but for the moment they have a clear enough hover state at least (though not a clear enough active one).
Definitely need some more thoughts though from @twbs/css-review :).
Closes #35293. Fixes #34123 (closed).