Change checkboard default colors to full opacity

It would be really useful if the default colors for the checkboard would have full opacity rather than being transparent. Currently, the provided pickers only support a white background, or else the checkerboard pattern for transparent colors is not visible.

https://github.com/casesandberg/react-color/blob/bc9a0e1dc5d11b06c511a8e02a95bd85c7129f4b/src/components/common/Checkboard.js#L21

For example, in my application I have added some custom styling to the `ChromePicker`, but the dark background leads to the checkerboard pattern becoming invisible. Would be nice to be able to make these kind of style changes, without having to write a completely custom picker 😞

Love the package btw!

Open src/components/common/Checkboard.js at the referenced line and inspect how the default checkerboard colors are defined. Confirm how the Checkboard is used by the ChromePicker, then change the defaults so the pattern remains visible over a dark background. Done means the default colors have full opacity without requiring custom picker code.

Written by the indexing model from the issue text.