You can change the appearance of the balloons in chat windows or turn off the balloons. You can also change the fonts used in a chat.
To view a chat as text, choose Show As Balloons from the View menu to remove the checkmark.
To change the balloon color and font used in your messages, choose Preferences from the iChat menu, then click Messages. Change the My Messages settings. Your changes affect the format of your messages on your screen, and on your recipient's screens, unless they have chosen to reformat incoming messages.
To override the font and color settings selected by other people and re-format all incoming messages using your preferred settings, select Use My Message Settings For Buddy's Messages.
You can also customize your chat windows by selecting an image to show in the background. Choose Set Chat Background from the View menu, then select an image file. To remove the background image, choose Clear Background from the View menu. The background image is visible on your screen only; it's not seen by those you're chatting with.