I am new to Drupal 8 and want to add some custom CSS in my bootstrap main theme, not sub-theme. I did not find any theme.info file in bootstrap theme directory to link my custom CSS like Drupal 7. Guide me how to add custom CSS in Drupal 8 bootstrap theme.
Bootstrap is designed for you to add extra CSS to a subtheme, not the main theme.
The Bootstrap documentation says this:
Warning You should never modify any theme or sub-theme that is packaged and released from Drupal.org, such as Drupal Bootstrap. If you do, all changes you have made will be lost once that theme is updated. Instead, you should create a subtheme from one of the provided starterkits (this is considered a best practice). Once you've done that, you can override CSS, templates, and theme processing.
Bootstrap even provides starter kits so you can easily make the theme yourself, plus step-by-step documentation.
Don't do on main theme instead create a sub-theme and change what you like. Here are some links that can help you set up Bootstrap sub-theme
You can use either SASS or LESS starterkit. Hope this helps you.
Though it's not a good idea to add your own CSS to a base-theme, you need to add it to