If you dont want to change the color of the text and just want to mark that item from the sidebar as selected (a grey overlay is placed over the whole sidebar item) you need to use the code below in the method where you build your sidebar items (eg. BuildNavigationBar or ConnectNavigationBar)
page.NavigationBar.ActiveSideReturnName = sideItemReturnName
Page.NavigationBar.ActiveSideSubReturnName = sideSubItemReturnName
where sideItemReturnName and/or sideSubItemReturnName is the returnId of the item that you want it to be marked as selected.
You'll find an example in the Demo project that comes with ABMaterial library.