Update buttons with Hover Effect

[makermelissa]

Add a hover effect on buttons by darkening 15%. This should be instant and match adafruit.com.

[Franqsanz]

Hello, @makermelissa, I would like to work on this!

[makermelissa]

Cool, thanks. It should be a pretty easy one.

[Franqsanz]

Great, if I think it's going to be easy.
Question should all buttons have the effect applied?

[makermelissa]

Fixed via #33.

[makermelissa]

Sorry I forgot to answer you question. Yes, it should be all buttons. If there are any that were missed by #33, we should get those too.