@baris Okay, I just made it work. I just copied useCustomHTML from Persona and added style type. It looks like this now.
{{{if useCustomHTML}}} {{customHTML}} {{{end}}} {{{if useCustomCSS}}} <style type="text/css">{{customCSS}}</style> {{end}}}and this code does not work anymore, just FYI:
https://github.com/pichalite/nodebb-theme-slick/blob/master/templates/header.tpl#L25-L27