Main target to set properties like date format or info type in input fields, so it can be used in documentation(by 3td software from your xml) at least and affect at visible component as maximum =)
Key=value pair would be quite sufficient to start, but I've seen only two pairs on one value customId="" and customData="" or I've mist something?
With UI theming I mean something like http://www.foreui.com/hl/feature_skinnable.htm
I'am sorry if it permitted to link on other project here.