Can you make this a Plugin Please

There are plugins that are ‘like’ this one, but this one is the best.
It’s simple, there aren’t a lot of options, it’s easy.

If not a plugin can you tell me which files I need to use to put this into other themes. This is perfect for a digital journal, but setup is rather boring. I’d like to look more like Pinterest.
I have a theme that looks like Pinterest, but of course I don’t know what files make this theme put the input box on top.

Thank you.