There are a couple of options here.
From the menus, you can select on "Format" and choose different font styles for the selected text:
This should allow you to edit the font, font size, alignment, text color, background color, etc:
The other option is to actually go into the HTML itself from the Tools menu.
However, I know that's slightly daunting, but that give you control to copy and paste all the HTML into a different editor:
For example, using http:// bestonlinehtmleditor.com/
And then copy and paste it back into Flexbooker.
Of course, if we can help, just let us know the changes you'd like and we'd be happy to make them, too!