Override :focus border styling

[mattcorner]

Would like to override the black :focus border colour but can't seem to work out how. Including in dropzoneClass styling doesn't work.

Originally posted by @mattcorner in #145 (comment)

[panz3r]

Relatable to issue #125 , the updated theme support should also solve this one.

@mattcorner , thanks for your feedback, I don't have a clear timeframe for the above issue but we'll look into it as soon as possible.