Is Grav right for my project?

First of all hello to everyone! I'm a beginner web developer and I am searching for a simple CMS to implement in my project. So I found Grav and I wanted to ask if it is the right choice for what I want to accomplish. Let me explain.

I want to develop a website the usual way (HTML/CSS/JS) and use a CMS for only one of its pages. More specifically, this page will contain a div with cards (thumbnail, title). The cards can be clicked and a lightbox will appear with extra info (main image, title, text, extra images/videos).

So I want a CMS to allow the client to manage only the content of that div, meaning he will be able to add/remove cards and edit the titles, text and images for each one. Is that possible with Grav or not?

Thank you!

Yup, this is all possible with grav. I think I even saw a theme that is similar to your project needs in the themes section.

Do I have to use a theme? Can't I just use the CSS of my project?

You certainly can use your own theme. However, it's always easier to modify an existing theme.