Try to add the following custom CSS into “Samsara Options > Custom Styles > CSS code”, If you selected blue large button, then you can use the following CSS
.vc_btn, .vc_btn_blue,
.vc_btn:hover, .vc_btn_blue:hover{background-color: YOUR COLOR;}
Othewise, you need to change .vc_btn_blue to the actual CSS selector.
By the way, If you want to customize some elements in the future, you can find the CSS selector with chrome developer tool. https://www.youtube.com/watch?v=V2aAEzlvyDc