There are a number of out of the box options for customising the appearence of the Messenger widget.
You can select the colors for your widget from a color picker or by directly entering the hexcode:
By default Messenger will use the colour pallet below:
But you can edit:
Text & Icon Color
You can change the icon that appears at the top of Messenger to an image file of your choice:
The default 'Get in Touch' can be edited to whatever best suits your brand.
You can also select the side of the page you wish the widget to sit on.