When you are creating or editing a site element that has text fields, you will be able to control the look and feel of those text fields in the Design Tab of the Hello Bar Editor.
Navigate to the Design Tab and then then open the Button Styling widget.
Within the Button Styling widget, you can edit the following features:
The background color:
The rounded corners of the button:
The border width and color of the button:
If you would prefer not to have a border around your button, just set the border width to 0px: