Buttons come in all sizes and shapes.
For use on darker backgrounds, you can just introduce inverted class to have your button flip its color to the opposite one.
<a href="#" class="mi-button mi-button-inverted" role="mi-button">Link</a> <button class="mi-button mi-button-inverted mi-button-outline">Button</button>
With some Sass magic, you can customize your buttons in a snap. Just include a color class declaration and set the color itself, and you're done.