make background black in bootstrap code example

Example 1: bootstrap 4 text color

<p class="text-primary">Text with primary color</p>
<p class="text-secondary">Text with secondary color</p>
<p class="text-success">Text with success color</p>
<p class="text-danger">Text with danger color</p>
<p class="text-warning">Text with warning color</p>
<p class="text-info">Text with info color</p>
<p class="text-light bg-dark">Text with light color</p>
<p class="text-dark">Text with dark color</p>
<p class="text-muted">Text with muted color</p>
<p class="text-white bg-dark">Text with white color</p>

Example 2: bootstrap 4 bg

<div class="p-3 mb-2 bg-gradient-primary text-white">.bg-gradient-primary</div>
<div class="p-3 mb-2 bg-gradient-secondary text-white">.bg-gradient-secondary</div>
<div class="p-3 mb-2 bg-gradient-success text-white">.bg-gradient-success</div>
<div class="p-3 mb-2 bg-gradient-danger text-white">.bg-gradient-danger</div>
<div class="p-3 mb-2 bg-gradient-warning text-dark">.bg-gradient-warning</div>
<div class="p-3 mb-2 bg-gradient-info text-white">.bg-gradient-info</div>
<div class="p-3 mb-2 bg-gradient-light text-dark">.bg-gradient-light</div>
<div class="p-3 mb-2 bg-gradient-dark text-white">.bg-gradient-dark</div>