Hello David,
I found your post as I was trawling through looking for a quick fix, rather than having to work it out myself. I don't normally look at chat module posts, usually I only reply to general and lessons.
However, I had similar issues and had to negate a background image that was used throughout the site and just have a white background.
Firstly can I say that you need to be editing your highest level styles_layout.css - ie the one that's in the theme folder you are using (mine for instance is in my own theme). You may need to turn off all the modules: chat section css in any other standard theme folders (in syles_layout.css in those) if these changes won't implement.
In styles_layout.css scroll down/search to get to the modules:chat section - all this css goes in there
To ensure the background is totally white throughout (or your choice of colour) add the following:
#message-messages,
#message-send,
#message-user,
#mod-chat-gui_header_js-chatinput,
#mod-chat-gui_header_js-jsupdate,
#mod-chat-gui_header_js-users
{
background-image:none;
background-color:#FFF;
}
Hope this helps