justauser Hi
That option is from very old version of JomSocial, with dark/light theme, it will now depend on the style of Joomla template you're using.
For Socialize template, you can go to Backend >> System >> Site Template Styles >> open style of Socialize template >> and change to theme you wish under 'Theme' tab:
There are also other templates supporting dark theme, pls check out the collection here: https://www.jomsocial.com/joomla-social-templates