Question: why main content has content-page css class

[lszczygielek]

Hi,
thank you for the theme, I like it.

I'm noob in css and web design/front-end stuff. I'm trying to learn a little bit. I see that main tag has content-page class, but in css files I can't find it. Could you answer what is a role of content-page?

Example from single.html:

<main class="content-page container pt-7 pb-5">

Best regards,
Łukasz