8.1.0-0J System Update of New 3DS have theme support, that System Update for launch day New 3DS is special about it since it have some titles from 9.0.0-20, also if I remeber well 9.0.0-X Home Menu should work without problems in 8.1.0-X, but you see this is done other way since the Home Menu is the one of 8.1.0-X effectively and no the one of 9.0.0-X.
Kernel9LoaderHax allows you to modify Home Menu directly. Though most other CFWs can if you do it on emunand instead. (can't modify sysnand system titles unless using Kernel9LoaderHax)