How exactly can i add stickers to items?

I have a big stock of items but still some of them are sold really fast, i wanted to add stickers to items out of stock ( as availability=0 than sticker appears on items)

  1. where and how in Algolia i can do this (i asked support but im new in IT and really hard to understand)
  2. I guess for sticker ill need to use CCS or you have other suggestions
    Thanks in advance