RGBA fallback fix

The 5% multiple returns a tenth of  the intended percentage - 4.5%
instead of 45%. Changing it to 50% corrects this. Also, replace the
`$shade: lighten` with `$darken: true`. Seems simpler that way IMHO.
Merge pull request #17 from ogbaoghene/RGBa-fallback-fix

RGBA fallback fix