The tools you have in Powerpoint or Keynote can be intoxicating. You can give a purple on green talk. You can have the new slide zoom in from any direction. You can make every slide with a different background. You can dazzle.
But the problem with being flamboyant in these ways is that it detracts from your amazing science. Save the finery for careful presentation of the concepts, what went before, and what you have discovered.
Just use black writing on a white background. For a talk, sans serif writing is the best. Look if up if you forget what sans serif is. This font is serif, preferred for text writing.
If you are as old as me, or even close, you might remember when we made slides for talks from actual film. One common technique resulted in white writing on a blue background. It is not as clear as black on white. Furthermore, it darkens the room, aiding sleep.
You talk should flow like an entrancing story where you magically make the listener to want the information right before you give it. An outline will kill that, so don’t have one. Build the plot, then summarize at the end. Have a narrative line where you build up curiosity. Then they’ll listen.
Sociobiology 
Agreed. Even better, use latex beamer or the new kid-on-the-block, R presentations (http://www.rstudio.com/ide/docs/presentations/overview) through Rstudio so you focus in the *writing* and not the flash.
In my view, your text shows up as sans serif, not serif. Probably depends on user settings? Good advice though.
Dave