In order to customize the WhatsApp contact button at the agent level, go to WhatsApp > All Accounts > choose an agent and click Edit.
In the Button Style box, you will see the design options to tweak how the button looks on the front-end. These placements include WooCommerce single product pages, and any page/post that you use this button shortcode on.
Button label: Personal greetings to your customers
Button style: round, square
Custom background color
Custom text color
So feel free to match the button design with your website color scheme.
The button customizer comes with a live preview so that you can see the changes you've made instantly.