Try this:
<div style="background:transparent !important" class="jumbotron">
<h1>Welcome!</h1>
<p>We're an awesome company that creates virtual things for portable devices.</p>
<p><a class="btn btn-primary btn-lg" role="button">Learn more</a></p>
</div>
Inline CSS gets preference over classes defined in a .css file and the classes declared inside <style>