@Cindy
The Official documentation on this topic is very sparse, u are right.

I have expirienced that changing the ui of a flow will not work without some browser digging apps like firebug.

For example using the FlowContainer Class will not give u automatically access to the basic properties of your flow style, like background-color, because these properties are rewritten within the flow page itself by other classes.

My goal was to simply change the background-color of the flow and this was my result (css declaration within an vf page which holds the flow element).

<style type="text/css">
        .FlowContainer {
            width: 99%;
        }
        body .bPageBlock .pbHeader {
            background-color: white;
        }
        body .bPageBlock .pbBody {
            background-color: white;
        }
        body .bPageBlock, body #bodyCell .bResource .secondaryPalette, body .secondaryPalette.bPageBlock, body .individualPalette .secondaryPalette.bPageBlock, body .bodyDiv .genericTable, body .genericPageBlockTable, body .bodyDiv .bSubBlock, body .bComponentBlock .bPageBlock, body .bMyDashboard .bPageBlock, body.rlHoverFrame .bPageBlock, body.subjectSelectionPopup div.choicesBox, body.lookupTab .secondaryPalette.bPageBlock, body.popupTab .secondaryPalette.bPageBlock, body.UserTagStatsPage .secondaryPalette.bPageBlock {
            background-color: white;
        }



<!-- clearing an known sf bug -->
        .interviewFormHeaderCell {
            border-bottom:0px;
        }
    </style>

With the FlowContainer Class from the sf documentation I was (only) eable to shrunk the window size of the flow (first class redefinition).

Changing the color of the flow detail window I was forced to open firebug and look which classes effect the header and the body of the flow detail (second, third and fourth class redefinition).

I'm not comfortable with this solution because I'm aware of the possibilty that the css redefinition will fail if salesforce change any of the class names in a future release. But actually I wasn't able to find any better way.

U asked for an example Cindy. This is mine. :)