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The best way to do this is to create a new file to store CSS customizations, a "style.css" or "custom.css" etc and define the new attributes for "Jumbotron":
.jumbotron-with-background {
    background-image: url(
    background-position:center;
    background-repeat: no-repeat;
    background-size: cover;
}
Hence it is loading this file after loading the Bootstrap CSS:
<link rel='stylesheet' href="/css/bootstrap.min.css">
<link rel='stylesheet' href="/css/style.css">
And finally put your customization in HTML...
<div class="jumbotron jumbotron-fluid jumbotron-with-background">
    ...
</div>
This applies to any other customization in Bootstrap, creating a new class to complement it instead of redefining existing classes.
