Bootstrap 4 Starter Page

This <div> is in a .container. It is responsive with break points.

The default grid in Bootstrap is 12 columns wide.

Layout is based on flex.

This section is inside a row and a . Or at least I think it does.

This is a .col-sm

col = column, sm = small with a breakpoint at 576px. If the browser window is less than 576px wide, the columns will collapse to a single, fluid column.

This is based on Bootstrap's Getting Started page

And check out the section on grid layout for more details.

This is another .col-sm

Check out Bootswatch for some pre-built colour schemes

bootswatch colour schemes

And here are some examples you can start with.