I've got it commented out here as I've reverted to v6.6.0, but this should do it:
.mx-dataview-controls .mx-button {
background: #65A2DA;
color: #fff;
/* margin-right: 3px; - add for button spacing in new footer 6.7.0 onwards */
}
Don't forget footers are no longer limited to buttons. You can easily solve this with, a layout grid, a table, or whatever your preferred way of dealing with this outside of dv footers happens to be.