Hi Christian,
Did you add paragraph tag on your buttons? That is what adding extra space on the bottom. Please add this custom CSS to remove it:
a.x-btn p {
margin-bottom: 0 !important;
}
If you want to add extra padding on top or bottom of the buttons, just add the following class custom-btn on each button class field. Then add the following CSS on Appearance > Customize > Custom > Edit Global CSS
.custom-btn {
padding: 1em !important;
}
Just add the class when you want to apply the padding.
Hope this helps.