bg-* utility classes code example
Example 1: bootstrap color
primary - #007bff
secondary - #6c757d
success - #28a745
danger - #dc3545
warning - #ffc107
info - #17a2b8
light - #f8f9fa
dark - #343a40
muted - #6c757d
white - #ffffff
Example 2: bootstrap background
.bg-primary
.bg-secondary
.bg-success
.bg-danger
.bg-warning
.bg-info
.bg-light
.bg-dark
.bg-white