Missing diashow option in Photos macOS Mojave

You can create slide shows in Photos 4 in the Projects section of the sidebar.

Hover the pointer over "My Projects", then click the "+" to create a slideshow.

Or use "File > Create > Slideshow > Apple".

For an instant slideshow view an an album, then click the blue "Slideshow" button at the top of the album.

Sorry, I misunderstood the question.

When you create a slideshow project, look for the options at the right border of the Photos window. You may need to enlarge the widow by dragging a corner, if it is too small.

At the right border are three buttons, one button to change the theme, one button to change the music, one button to change the duration of the slideshow.

For an instant slideshow, played directly from an album, you should be seeing a drop-down panel, when you click "Slideshow", where you can change the theme and the music:

Which kind of slideshow are you trying to create? A slideshow project or an instant slideshow from an album?

Which system version do you have installed?

On the newest version of Mojave macOS 10.14.6 I can see the options in dark mode as well as light mode. But the contrast is is not very high. If you can barely see them, you may want to check, if you have Night Shift enabled as well in the System Preferences > Display. The Night Shift can make the contrast worse, because it is changing the colors slightly. And I noticed, that some controls can become barely visible, if System Preferences > Accessibility > Display > Contrast is modified. If you changed any of these settings, you may want to revert them to the defaults, if you notice missing controls in Dark Mode or Light Mode.