I am not sure if most of you already know this but I tried and found
this working.


In portal listing one has the ability to display information in
various formats and one of the formats permit you to group the
listing under group headings.

To control the font characteristics of the group heading you have the
following style

.ms-portaltitle {
font-family: Arial;
font-size: 11pt;
font-weight: 700;
Color: FFFFFF;
padding-bottom:10px;
padding-left:0px;
padding-right:0px;
}


I tried adding a line to this style

background-color: #98DCF5;

and the result was a nice band of background color stretching across
the length of the group heading text.

This in my case helped users see the group headings more distinctly
in the portal listing