Nuno,
While there is no way to conditionally change the font applied to a variable, there is a little trick you can use.
The idea is to place two variables in your text box, next to each others. Then each variables can have a conditional expression. They will display some text if the expression is true, and nothing if it's false. After that, you can go in the text box, and apply a different font and style to each of the two variables.
This means that on each record, only one of the two variable will be displayed, and though they might have the same value, they will have a different font, thus simulating a conditional fonts change.
Hope that helps!
Regards,
Raphaël Lalonde Lefebvre