I have a similar problem:

I want to change the background color of the chatbox and the message-box itself.
Does anyone know which values  I have to change ?

thanks in advance, Jochen

Jochen. This is a huge one. You will see that in the attached file. The trick is to keep the css changes from affecting other areas since the chat, mailchat, and messages page all use some of the same elements. There are several elements that have to be styled. You can use the colors that I used, or change them to your own, but you will need to keep in mind that the colors you use may conflict with the text, or the icon colors.

Thank you very much Darryl ,  I`try those settings, br, Jochen

Hey Darryl, you made my day !  It`s working !!!  Thank you very much !!! br, Jochen

The admin panel layout comes from outside the theme "pulls from the Simplicity theme ", so no it doesn't change the admin panel components.  Not really much need for that since the only one that sees that area is the admin. The objective is to give the user a visually pleasing experience. 

Yea. I played around with it, but it looks like you would have to add the css to the admin html file in the Simplicity theme's master pages. I didn't try that, but they do already have css defined there.

Yes, I added snipplets of your code via admin panel to the Simplicity theme.
Works perfect !

Where do i exactly put this code that Darryl B give us?

You would paste it into the custom css box for the theme in the admin panel. admin / appearance / customize; then click on the css tab. You will see the custom css box under the base css display.

Thanks Darryl B, For the message color/text fix!!