BuddyPress.org

@raminjan install the Firebug add-on for Firefox, right-click on the element you want to change, then add the changes to your stylesheet.

The current styles for the updated message

div#message.updated {
clear: both;
}
div#message.updated p {
background-color: #8FF57A;
border-color: #80CF70;
color: #1A6A00;
}
div#message p {
background-color: #F4F4F4;
border: 1px solid #DDDDDD;
color: #555555;
font-size: 14px;
margin: 20px;
}

So use same selectors and change the property/value as you want it acoordingly in your theme’s stylesheet.