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Since the majority of carriers especially the major ones in the States have not officially released the 4.5 OS, not an awful lot of people are using it and therefore not creating themes for it. Also they may not be creating them because if they are using an unsupported OS they may not want to take the chance of damaging their phone by testing themes. Just wait a little while for the OS to become more widely used and I'm sure then you'll be able to find a theme that's to your liking.