Hello Kerim,
.urSTSSubtot CSS class is defined in /sap/public/bc/ur/Design2002/themes/sap_tradeshow/ur/ur_ie6.css file (for IE). I believe that if you would change in the ur_ie6.css it than the totals style will be changed for every report.
Thus if you need to change the totals color only for a single report it would be better to insert the Below code in your WAD template through Web Application designer.
...
<head>
...
<style>
.urSTSSubtot{background-color:BLUE !important}
</style>
...
</head>
...
K.