Hey Galini,
please try this CSS:
#page-holder-21 .page-content .su-button { background-color: #ffffff; }
#page-holder-21 .page-content .su-button:hover { background-color: #ffffff; }
replacing the #ffffff in both cases with the needed color in HEX notation; the second declaration is for the hover state. In case you wish to modify the background color for all shortcode-generated buttons, you can use this instead:
.su-button { background-color: #ffffff; }
.su-button:hover { background-color: #ffffff; }
Sincerely,
Andrii / Satori Studio