Adding custom CSS to themes required a plugin previously. Now you can do it with the customizer and additional directions are available at the link above.
Many times you want to show posts from certain categories on pages with additional context. This plugin has a huge amount of flexibility especially when blended with some of the CSS that we've discussed on other pages.
If you choose a theme that uses Bootstrap you have a variety of built in options that make extending that theme easier. There are also plugins like this that give you shortcodes built for Bootstrap.