One of the great features of Ubuntu's default instant messaging application 'Empathy' is the support for 'Adium Message Styles'. These let you theme your messaging window in a variety of ways. The only draw back is that most of the ready-made Message Styles available online are made for Adium (the Mac OS X version of 'Pidgin') and don't always work well in Empathy on Linux. But Linux-made variants are available - such as this 'Ambiance Adium Message Style' by franksouza18 3.