An easy way to make themes for later levels is embracing the "boon" design of DMG 2.
Whenever you feel the need, you can just add a boon which gives a power and an item bonus associated with that. (or even an untyped bnus)
The only thing preventing that is the unability of both char generators to modify items. (Boons are implemented and can even add things like a new power, see corellons boon of arcane might... although there is a bug which makes multiple boons of arcane might useless)
Sorry, for the necro, but loremaster.org no longer exists. Are the blog articles on themes available anywhere else?