I attempted to replicate your issue by integrating the complete vertical timeline demo along with the bootstrap styles.
After analyzing in your web inspector, it was evident that when you include the bootstrap styles before the timeline styles, the order of applied stylesheets for elements like <h2>
is: styles.css, then reset.css, and finally the bootstrap CSS:
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="stylesheet" />
<link href="https://codyhouse.co/demo/vertical-timeline/css/demo.css" rel="stylesheet" />
<link href="https://codyhouse.co/demo/vertical-timeline/css/style.css" rel="stylesheet" />
<link href="https://codyhouse.co/demo/vertical-timeline/css/reset.css" rel="stylesheet" />
<header>
<div class="cd-nugget-info">
<a href="https://codyhouse.co/gem/vertical-timeline/">
<span>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="16px" height="16px" viewBox="0 0 16 16" style="enable-background:new 0 0 16 16;" xml:space="preserve">
<style type="text/css">
.cd-nugget-info-arrow{fill:#383838;}
</style>
<polygon class="cd-nugget-info-arrow" points="15,7 4.4,7 8.4,3 7,1.6 0.6,8 0.6,8 0.6,8 7,14.4 8.4,13 4.4,9 15,9 "/>
</svg>
</span> Article & Download
</a>
</div>
<!-- cd-nugget-info -->
<h1>Responsive Vertical Timeline</h1>
</header>
<section class="cd-timeline js-cd-timeline">
<div class="cd-timeline__container">
<div class="cd-timeline__block js-cd-block">
<div class="cd-timeline__img cd-timeline__img--picture js-cd-img">
<img src="https://codyhouse.co/demo/vertical-timeline/img/cd-icon-picture.svg" alt="Picture">
</div>
<!-- cd-timeline__img -->
<div class="cd-timeline__content js-cd-content">
<h2>Title of section 1</h2>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Iusto, optio, dolorum provident rerum aut hic quasi placeat iure tempora laudantium ipsa ad debitis unde? Iste voluptatibus minus veritatis qui ut.</p>
<a href="#0" class="cd-timeline__read-more">Read more</a>
<span class="cd-timeline__date">Jan 14</span>
</div>
<!-- cd-timeline__content -->
</div>
<!-- cd-timeline__block -->
/* Additional sections ommitted for brevity */
</div>
</section>
<!-- cd-timeline -->
<script src="https://codyhouse.co/demo/vertical-timeline/js/main.js"></script>