Pro tmavé pozadí:

#sp-main-body .container {
  background-color: rgba(255, 255, 255, 0.7) !important;
  padding: 20px !important;
  border-radius: 0 !important;
}

Pro světlé pozadí:

#sp-main-body .container {
  background-color: rgba(0, 0, 0, 0.7) !important;
  padding: 20px !important;
  border-radius: 0 !important;
}