do you mean in the text body or the subject?. If you click on post reply you get more options and can change it there rather than using quick reply as that uses a more uniformed text as standard.
if its to change it like to a differnt type and colour, size for good you cant do that. you just have to change it each time you post!! like this highlight it then click the A with the colour underneath then pick a colour then if you want to change more click go advanced and there is options there to change the font like nicky said