There are a number of out of the box options for customizing the appearance of the Messenger widget:

Colors

You can select the colors for your widget from a color picker or by directly entering the hexcode:

By default Messenger will use the color palette below:

But you can edit:

Primary Color:

Secondary Color:

Text & Icon Color:

Icon

You can change the icon that appears at the top of Messenger to an image file of your choice:

Greeting Title

The default 'Get in Touch' message can be edited to whatever best suits your brand.

Widget Postition

You can also select the side of the page you wish the widget to sit on.