Find the Live Chat settings under the 'Live Chat' tab in the Website -> Settings section.

These settings control the live chat feature that customers can use to contact you through your website. To get started, first enable the 'Website Chat' setting.
You can choose a chat header and also type in what you'd like the customer to be greeted with automatically with the 'First Message Text'.
The 'Offline Text' setting is what will be sent to the customer when they have sent you a message and you have not responded within 2 minutes.

Below are all your aesthetic choices, using colour schemes previously chosen under Website -> Colours.

Click 'Update Settings' when you've finished and have a look at your website to check the changes.
Note: You can send live chat messages to test the process yourself.