Header:
I'm not sure what you want to change in the footer. ie BG, Font, Font Color,
etc.
If you want to just change the BG/FONT goto View/Masters/Slide Master. Just
select the footer and do what you want.
Anything you do on the Slide Master will affect all slides.
Font Color:
The best way to handle all colors is not through the Master but by using
Color Schemes. Pick and apply a color scheme. You can also edit that color
scheme to be the colors you want.
The nice thing about color schemes is it allows you the opportunity to apply
different colors for the same slides in the future.
