You can change the font, color and size of any of the titles. Just double click on the title in the timeline, and you're editing options will open up in the Adjust window. There are only 10 font options pre-installed in iMovie, but at the bottom of the list you can select Show Fonts... This will open up your computer's font library, and you can use anything that's installed there.
One nice feature, design-wise, is that in the titles that are two lines, you don't have to use the same font, size or color. This gives you a lot of freedom for making creative titles for your videos. Unfortunately, you can't move the titles around on the screen, so you're stuck with the predetermined location.