Leda_Atomika

All good - I found a solution:
.sqs-block-content .lightbox-handle {
 background: #B1C7B4 !important;
}
.sqs-block-content .lightbox-handle:hover {
 color: #fff;
 font-weight: 400;
 background: #B1C7B4;
}
and for the Submit button:
/* Target the submit button and change the background color */
.button.sqs-system-button.sqs-editable-button.sqs-button-element--primary.FecjBqAV0ViY3ijU58R1 {
  background-color: #B1C7B4;
  font-family: "Poppins", sans-serif;
  text-transform: uppercase;
  color: #fff;
  padding: 8px 20px;
  border: none; 
}
and I also changed the font of the form to match the rest of the site (Proxima Nova):
/* Form Title */
.form-title {
  font-family: "Proxima Nova", sans-serif;
  font-size: 17px !important;
}
/* Labels for First Name, Last Name, and Email */
.field-list {
  font-family: "Proxima Nova", sans-serif;
  font-size: 16px;
}
/* "Required" text in labels */
.description.required {
  font-family: "Proxima Nova", sans-serif;
}
/* Label and Input Fields */
label.caption {
  font-family: "Proxima Nova", sans-serif;
}
Hope this helps someone else to customize this...