when using the following syntax with React 15.x

Label {}
Label:prop(mode == 'edit') {}

<Label mode="edit"/>

React throws the following error:

Warning: Unknown prop `mode` on <div> tag. Remove this prop from the element. For details, see https://fb.me/react-unknown-prop

If I understand correctly what is being said here, you should be able to remove unknown props. Basically, that means that the syntax you are currently using should be deprecated.
I would suggest keeping to the variant prop:

<Label variant={{mode: 'edit'}} />

Tell me what you think.

EDIT: On second thoughts, the syntax is still very useful in many cases. For instance, you could choose to change the style of the component when onClick prop is set. I think it should be clearer in the doc that the library does not whitelist the props, and that you should use this feature only with props the component accepts.