Add caption formatting to css.

chrome/common/extensions/docs/static/sass/_typography.scss

 /* =============================================================================
    Typography
 
    To achieve a pleasant vertical rhythm, we use Compass' Vertical Rhythm mixins
    so that the line height of our base font becomes the basic unit of vertical
    measurement. We use multiples of that unit to set the top and bottom margins
    for our block level elements and to set the line heights of any fonts.
    For more information, see http://24ways.org/2006/compose-to-a-vertical-rhythm
    ========================================================================== */
 
@@ skipping 55 matching lines @@
 //   }
 // }
 
 p {
   margin: rhythm(1) 0;
   &.noindent, &.caption & {
     text-indent: 0;
   }
   &.caption {
     text-align: left;
+    margin-top: 9px;
+    font-weight: bold;
     .lightbox & {
       color: $white;
     }
   }
 }
 
 a,
 a:link,
 a:visited {
   color: $link-color;
@@ skipping 170 matching lines @@
   line-height: 1.4;
   text-shadow: 0 1px 0 #fff;
   font-family: Arial,Helvetica,sans-serif;
   display: inline-block;
   padding: 0.1em 0.6em;
   margin: 0 0.1em;
   white-space: nowrap;
   box-shadow: 0 1px 0px rgba(0, 0, 0, 0.2), 0 0 0 2px #ffffff inset;
   border-radius: 3px;
 }