if you want to modify just the timestamp, you're going to have to go into the js/chat.js file and find the function that adds messages to the chatlist.. iirc it's called "addMessageToChatList" (I'm not currently looking at the code) and then find the time variable and give it its own div tags, that way you can add an id and then set the text colour for that div in your css files. Or, you just set the colour as an inline style so it's the same in all themes, but that could potentially clash so I wouldn't advise it.