I think it's necessary to have at least option like font size which can be changed by user, especially for phones whose screen is small.
Further more, if we can choose to hide the time stamp, the user name (in private chat), we would be able to focus on the messages themselves.
Wayne Walker commented
Yes, the UI needs to be somewhat customizable.
* font face
* font size
* choice of timestamp format
* visible / hidden for
* channel list