No background showing in pdf

Somehow the dark background of my presentations turn to white when I convert them to PDF. Even older PDF's with a dark background at the time now all have a white background. Because I use white fonts for titles, this is not userfriendly at all for my students.

I thought I had found a solution: you should uncheck 'Replace document colors' in the accessibility options. Maybe it's because of the new interface of Adobe, but even by using the Help option I can't find this option. Can anybody please let me know what I can do to keep the dark background?

Thank you!