This is a drop-in replacement for meteor's core stylus package. It allows mixins. To use, simply create a lib/mixin folder and put your mymixin.styl into it. Any .styl in a folder named mixin will be assumed to be a mixin.
Have some files like these:
/myapp/client/css/lib/mixin/colors.styl
/myapp/client/css/lib/mixin/shorthand.styl
/myapp/client/css/mystyle.styl
/myapp/client/css/anotherstyl.styl
Install with meteorite: https://atmosphere.meteor.com/wtf/app