Blue round (story panel button)

Hello,
I wanted to change the size of the button (story panel button), but I see a blue circle under the button "Create a publication" when I modify the class "

#es .es-story-panel-button__shape{    

    -webkit-flex: 0 0 52px;

    -ms-flex: 0 0 52px;

    flex: 0 0 52px;

    font-size: 52px;

    border-radius: 52px;

    border: 2px solid #fff;

    width: 52px;

    height: 52px;  

}

What is this round?

Best regards.

Eric