The danger of gradient fills
What this presenter did was to fill every shape with a gradient fill that moved from black at the top to white in the middle and back to black at the bottom. Then he put text in the shape. Of course it was impossible to select a text color that had enough contrast with both the black and the white background, so most of the text was almost impossible to see.
Am I suggesting we never use gradient fills? No. I use them in the background of my own slides. But here is the way I think they work best. The two colors should be fairly close to each other so that when the color changes, there is no significant difference in contrast with any text or shape that may be on top. For example, having a gradient from black to navy blue would work well. I also suggest only one gradient, i.e. go from one color to the next but don't go back to the first color again, it is too busy.
If you are going to use gradient fills, keep them simple and maintain the ability to see whatever is placed on top of the fill by selecting appropriate colors.