When you theme with a theme from an older ROM, most of the time it will only break visual aspects that were in the new version. So let's say you flashed the transparent notification bar for ths 6.0. You would lose the aokp notification bay and have the touchwiz looking one from ths.. That is an extreme scenario, and i don't suggest doing it. But the most i have experienced from flashing themes from older versions was visual reversions of that prior ROM, and some of the things added sense that earlier ROM were broke. Like if ROM had a flashlight mod on lockscreen, but theme from earlier rom did not.
But no matter what, nandroid, and wait till odin is handy
Only you can take responsibility for breaking you're phone
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