Example:
.bg-primary
.bg-primary-subtle
.bg-secondary
.bg-secondary-subtle
.bg-success
.bg-success-subtle
.bg-danger
.bg-danger-subtle
.bg-warning
.bg-warning-subtle
.bg-info
.bg-info-subtle
.bg-light
.bg-light-subtle
.bg-dark
Opacity:
This is default success background
This is 75% opacity success background
This is 50% opacity success background
This is 25% opacity success background
This is 10% opacity success background