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Images in Bootstrap are made responsive with .img-fluid .
This applies max-width: 100%; and height: auto;
to the image so that it scales with the parent element.
Responsive image.
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$thumbnail-padding: .25rem;
$thumbnail-bg: $body-bg;
$thumbnail-border-width: $border-width;
$thumbnail-border-color: var(--#{$prefix}border-color);
$thumbnail-border-radius: $border-radius;
$thumbnail-box-shadow: $box-shadow-sm;