We are amongst others waiting for CLDIG-I-329 which should give us the option to CSS the iframe in the web chat widget. I can tell from other posts that this should be released in q4 18 or q1 19 but i cant find it on any roadmaps. Is q1 19 still eta?
This means that we will make chat APIs publicly available and allow customers to build their own widget from scratch. Since you'd be building your own widget from scratch, at that point it's up to you to also design your own CSS.
If you do not wish to build and maintain your own widget from scratch, this feature won't give you the ability to edit css in the existing PureCloud widget.
For those customers who would prefer to use a chat widget that Genesys builds and maintains, while being able to customize colors, we have plans to provide a new chat widget. The dates for when the new widget will be readily available are not set yet.