Add colors for the background chromatics at different opacity levels to JSON files

[0xd3e]

The website mentions the use of different opacity levels for the background chromatics.

Since not every application is able to handle opacity, it would be nice to have the colors available via the JSON files. I'm not quite sure how this can be calculated, since the background color isn't pure white or black.

Here is an example to further clarify what I'm looking for:

The color bgDarkBlue is #7F99A7 at 50% opacity on the background of the light color palette.
So it would be nice if the JSON files would include a color name like bgDarkBlue50 and its value would calculate to #7F99A7 when running through the lunarize script.

[dfoxfranke]

Sounds sensible; I'd take a PR for this. gencss.py should probably ignore these entries, since anything that can process CSS can probably handle transparency and the CSS files are already pretty big.