The documentation for Bootstrap actually provides an illustration featuring a prominently styled title here. You can utilize a heading tag with pre-applied bold font-weight and use the "panel-title" class for adjusting size and alignment. Below is the specific example from the documentation:
<div class="panel panel-default">
<div class="panel-heading">
<h3 class="panel-title">Panel Title</h3>
</div>
<div class="panel-body">
Panel content
</div>
</div>
If you have a more general inquiry, such as modifying default styles, you can apply any traditional CSS methods, for instance, using a strong tag:
<div class="panel panel-warning">
<div class="panel-heading">
<strong>Panel Title</strong>
</div>
<div class="panel-body">
Panel content
</div>
</div>
Alternatively, you can incorporate an inline style:
<div class="panel panel-warning">
<div class="panel-heading" style="font-weight: bold">Panel Title</div>
<div class="panel-body">
Panel content
</div>
</div>
Or (ideally) by assigning another class from your external stylesheet:
/* Custom CSS file */
.bold {
font-weight: bold
}
<div class="panel panel-warning">
<div class="panel-heading bold">Panel Title</div>
<div class="panel-body">
Panel content
</div>
</div>
If you are considering making all panel titles bold, you should either 1) duplicate the bootstrap definition for "panel-heading," make the desired changes, and place it in a stylesheet with higher priority (lower on the page) to take precedence, 2) alter the existing bootstrap code (less recommended), or 3) compile your version using less by importing the primary bootstrap less files and supplementing them with your modifications. The latter option is the most versatile and arguably the most favorable, albeit more labor-intensive. However, this method allows you to "inherit" styles (as you mentioned) as less enables you to incorporate other classes within your own class, like so:
.bold-heading {
.panel-heading; // Apply base panel-heading styles
font-weight: bold; // Override the properties you wish to alter
}