bootstrap 4 bg-primary 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 4 bg
.bg-gradient-primary
.bg-gradient-secondary
.bg-gradient-success
.bg-gradient-danger
.bg-gradient-warning
.bg-gradient-info
.bg-gradient-light
.bg-gradient-dark