The font size of Empathy in Ubuntu 10.04 is too small to get a good visual effect. Here is the way how to adapt it to your favorite type and size.

1. sudo gedit /usr/share/adium/message-styles/ubuntu.AdiumMessageStyle/Contents/Resources/main.css
2. Change the font type. Search the keyword “font-family”, change it into your favorite font type, e.g. font-family: WenQuanYi Micro Hei.
3. Change the font size. Search the keyword “.message”, locate the first result, change it to a bigger value, e.g. font-size:14px.
4. Exit and restart application Empathy. It should work right now.

Enjoy your chat with friends using your favorite font type and size.

Reference: http://hony.xiaokunyu.blog.163.com/blog/static/1295107592010436361089/