What is now used to quickly and adaptively write online stores?

Question

css

Good day to all! I wrote all my work on pure code using google mdl. The backend also sawed exclusively on pure php, working with the database through the console. Now, having gained some experience, I came to the conclusion that this is not correct.
The main tasks from frameworks or cms:
1. Convenient and not CURVED adaptive layout
2. Simplification of work with the backend
Googling came to the conclusion that there are millions of options. Need your advice on the best way to use it.
I write websites naturally with ready-made designs, both for desktops and mobile devices. The main problem now is a crooked adaptive

Answer 1

Bootstrap, grid, flex?

Answer 2

Responsive layout, are these requirements for a framework or cms?
I hope you meant css framework.
hostcms is lightweight and flexible.