I am a fresh front-end developer and I want to start working with Magento 2.
I would like to know what are the required skills to start working on the Luma theme as a first step?
With what grid is Magento 2 working? CSS grid or bootstrap flex?
Welcome to the community.
You can follow the devdocs by Magento.
For Frontend Developer Guide follow this link.
If you are interested in theme designing than you can follow Theme development guide.
To include external bootstrap file Include Bootstrap you can follow this tutorial.
Magento 2 uses LESS to know more how to compile them follow their guide CSS and LESS
Magento 2 does not use any bootstrap it has its's own classes and grid.
Magento 2 now comes with Less and Luma theme which is well implemented.
Magento 2 Common layout customization tasks
Magento 2 Mage 2 Tech Stack
Update : Learn about Magento Modes i.e default/developer/production
Always be in developer mode while working at code level to set developer mode use
Change To Developer Mode Using Command Line
php bin/magento deploy:mode:set developer