MS powerpoint question: anyone know how to...

...add design to a powerpoint [b]without[/b] messing up the font and font sizes already on your slides. I have finished a powerpoint. When i hit the design buttons to add colors, they have their own fonts and they override mine. I don't want to have to fix each font and size, reposition all my wording and pictures on each slide because of this. I've already google. I don't see it.
😕

I would just do a solid backgound on all slides but it would look too boring