BootstrapVue uses Bootstrap V4.3's SCSS for styling and variants.
You can either custom theme Bootstrap V4.3's SCSS (via SCSS variables) to change the default colors for each variant, or you can create custom variants in CSS/SCSS.
Navbar's use the classes bg-<variant name> to define the background color.
You could create a class bg-foobar which has CSS background-color: #112233;.
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