The resource bitmap packs are not the same as the visual styles. All the visual styles (Luna Royale, Luna Element, Zune, etc) are installable and selectable, regardless of the resource pack (MCE, Darkside, etc) that you choose during installation. The resource pack specifically affects icons, pictograms, log screens and so on -- the visual styles (the "themes") specifically change the look of the windows (frames, buttons, etc). Installing a resource pack involves patching a lot of system libraries and executables to replace the old built-in bitmaps with newer ones. That's why you can't change a resource pack with another without having to reboot. All you can do on the fly is switching the visual style.