Yes.

To change the text font (e.g. colour, size, family and etc) of your chat link (a chat button with Text Link type), you need to manually modify your live chat code before it is pasted onto your website.

Followed is a sample about how to modify your live chat code. You only need to add the style attribute style="font-weight:bold; font-family:verdana; font-size:20px; color:#339999;" after  <div id="comm100-button-xxx"