dark mode issue

I have the newest zoom update (5.11.0), and I really need to switch to dark mode because light mode is very straining to the eyes, especially with a huge iMac screen close to my face. however, the "dark" option is not available. in fact, the "theme" category is nowhere to be found in my settings on general in zoom.  where can I enable dark mode here in this?

I use macOS High Sierra 10.13.6. pls, I need a quick answer to this, or i'll probably go blind after some time.