sitcky text plugin?

hey guys, I have a question.

I’m creating a blog and I’m still in the building process.

I found s feature on a theme that I want to find a plugin that does the exact same thing.

Here’s the thing : https://pasivniprihodi.com/blog/

You see the red square with the text inside it, that is sticky only on the blog page and is always on top. This is a part of the theme itself.
It’s not a post. It’s a message/call to action and I want it to be on top of the blog page always.

I need a plugin that does that but to every theme I install since I’ll be changing the theme, cause this is just a starting theme till I find one i really like.