Attempt to replace button custom CSS with Adwaita button style
We have some custom button CSS in data/nibbles.css which is causing us problems. As far I remember is mostly copied from Adwaita, so it will never be kept up to date at all times. We should consider removing it and applying the theme's default button style.
However, we need to see how well that would work when the user has a custom theme installed, as I assume that means the button will look different as well.