Both 'leopard print' and 'ninja' themes sound like awesome bedroom designs to me. Perhaps it's worth sitting down with your mum and explaining exactly why you are interested in doing your room up this way.
If your mum still thinks that a 'ninja' theme sounds too wild, perhaps one idea would be to go for a toned down 'Japanese' theme as a way of stealthily sneaking the 'ninja' element back into your design.
Japanese bedroom theme
One thing that might convince your mum to let you go ahead with the Japanese theme is that Japanese bedrooms tend to be rather minimalistic. Whilst this doesn't necessarily mean your mum will immediately approve, implying that you will only need minimal furniture will definitely sit well with your mum's credit card balance.
Things you may want to include in your Japanese bedroom design:
- Tatami mats
- Low bed (the lower the better)
- Shoji screen
- Japanese lantern
- Geisha-inspired art work
Turning your bedroom Japanese
The way to approach the design of your bedroom is to start from the ground and work your way up. Tatami mats, which are usually made of rice straw, are a great base for your room, and have the added advantage of feeling great to walk on with feet!
Your bed will be your centerpiece, therefore it's worth spending a little extra time making sure you pick the right one. Stores like Ikea usually have a number of modern-looking designs that will do the trick- remember that with as far as Japanese beds are concerned, the closer to the ground, the better.
Neutral colored dressers flanking the bed will complete the basis for your room. And all that'll be left is to accesorize!
Anything with a cherry-blossom or bamboo print will add to the authentic feel, as will items like lanterns or some Japanese-inspired art work.
Including a shoji screen could be a nice way to partition your room, and it could also double as a screen for hiding the mound of clothes piling up in your room next time your parents complain about it!