I see. Not sure if I’m understanding right, but I would’ve guessed that one of these types here are what you’re looking for. The Slide In sounds like the sidebar sticky one. A Sticky bar is the top of the page, and Inline could be anywhere within the content, I believe.
As far as CSS customization, I’m really not sure. I’m guessing you’d be better off using a 3rd party software, like Sumo or Opt-In Monster and then integrating with CK. I’m not very familiar with these other options though.