I think every theme is based on Foundation. It’s how a forum looks by default. There is no need for your theme to change a button color that already looks like it should by default. These defaults change now, which is why it affects all themes - even the Horizon theme is affected:
You can see that the header is thinner, the chat icon is darker, and the button is no longer filled.
What you just did is the advantage of the upcoming changes system: you can see how this affects your forum and act based on that before your forum has to use the new defaults.