In FX Draw, you can select any number of objects and quickly change the fonts of both normal text and equations. To begin, select the objects you want to change (or type Ctrl+A to select all objects).
In FX Draw's Edit menu, you will find font styles entries.
Note that you can quickly select one of the four saved font styles using the Alt key combinations and this is how you are likely to end up using Font Styles. For the moment, we will choose Change Font of Selected Objects...