The example is using Actuate 11 SP2, BIRT 2.6. However the code below will also work in earlier releases.
var t;
if (params["pTheme"].value == "New") {
t=reportContext.getReportRunnable().designHandle.getModuleHandle().findTheme("DynamicTheme.NewTheme"); reportContext.getReportRunnable().designHandle.getModuleHandle().theme = t;}
else {
t=reportContext.getReportRunnable().designHandle.getModuleHandle().findTheme("DynamicTheme.OldTheme"); reportContext.getReportRunnable().designHandle.getModuleHandle().theme = t;
}