I have a logo with a red heart with stylized letters inside. Surrounding this are text which currently are black. However, I will be making a Keynote in a Black Theme and need the exported image to have white letters.

My "transparent background" looks white. Is there a way to make that any other color so I am able to see the white letters?

When I export as a .png and open it in Preview, it has a transparent background. So is there a setting that allows me to adjust that transparent background to show some other color than white?

Sorry, I do not seem to be able to attach the file.

Usually I'd go document preferences, page color and get rid of the transparency setting.

In Document Properties, the checkerboard option will show transparent areas.