What could be the property of the back of this button in the Niara theme?

[MaxStrax]

Hello,

I'm customizing the appearance of the 'Continue shopping' and all related buttons:

• .btn.btn-lg.btn-default
• .btn.btn-lg.btn-default.btn-full

.btn.btn-lg.btn-default,
.btn.btn-lg.btn-default.btn-full {
  border: none;
  background-color: transparent;
}

But, I can't find the property that makes still appearing what is shown in the pic below. Some kind of transparent layer with a black/grey gradient around it.

It's only shown when the user clicks on the button.

Do you know what propierty this element is?

 

Edited August 12, 2023 by MaxStrax