In your case about the buttons colors you needed that:
body .sppb-btn.sppb-btn-secondary, body .sppb-btn.btn-seconday,
body .sppb-btn.btn-primary, body .btn.sppb-btn-secondary,
body .btn.btn-seconday, body .btn.btn-primary {background: red;}
body .sppb-btn.sppb-btn-secondary:hover, body .sppb-btn.sppb-btn-secondary:focus, body .sppb-btn.sppb-btn-secondary:active, body .sppb-btn.btn-seconday:hover, body .sppb-btn.btn-seconday:focus, body .sppb-btn.btn-seconday:active, body .sppb-btn.btn-primary:hover, body .sppb-btn.btn-primary:focus, body .sppb-btn.btn-primary:active, body .btn.sppb-btn-secondary:hover, body .btn.sppb-btn-secondary:focus, body .btn.sppb-btn-secondary:active, body .btn.btn-seconday:hover, body .btn.btn-seconday:focus, body .btn.btn-seconday:active, body .btn.btn-primary:hover, body .btn.btn-primary:focus, body .btn.btn-primary:active {
background: #222;}
With the dark header, I also used extra CSS line.
Yes, I already added & used on your site :)
All is as you wanted.