I just updated to 5.1.2 and I don't see the 'Theme' tab in the options window. There doesn't appear to be a Dark Mode option anymore, Or is it just me?
It's still there. Are you sure you're running V5 and not an older V4 release? In V5, you just go to Tools > Options (or Simplify3D > Preferences on Mac), click on the Theme tab, and you can choose between the default or dark mode themes right at the top.