Before starting a presentation design project, I need some basic guidance from my clients: dark or light background, custom fonts or not, Mac or Windows. Useful information.
A few times, I made the mistake of asking design (not content) input on specific slide elements: this way of putting pictures or that way, this type of titles or that, black & white or colour. It somehow did not work. As a designer you need to select the entire design approach in a consistent way.
I sometimes see something similar in interior designs of houses: individual elements look OK, but the whole composition together does not make sense.
Mixing and matching gives mediocre results.