resolves #338 rescale color palette to use darker shades of gray

[slonopotamus]

This commit also removes Kindle-specific color adjustments so minor color changes are expected on Kindle

[slonopotamus]

@nscheer could you please test this on your Kindle?

[nscheer]

@nscheer could you please test this on your Kindle?

Yes, gladly. Is there a specific test document I can build or can you provide one?

[slonopotamus]

@nscheer Please, see attached archive with before/after Mobi files of the same sample book:
shades-of-gray.zip
GitHub rejects *.moby attachments for some reason.

[slonopotamus]

@craigscott-crascit Could you also give this PR a try?

[mojavelinux]

I know @cexbrayat is also interested in the color palette being one of the ones who originally reported an issue on this topic.

[craigscott-crascit]

Initial results look good, I'm just waiting on feedback from one other device user.

[nscheer]

@nscheer Please, see attached archive with before/after Mobi files of the same sample book:
shades-of-gray.zip
GitHub rejects *.moby attachments for some reason.

Checked both files this morning. On my Kindle Oasis 2020 I'm unable to spot any differences.
Looks good to me!

[slonopotamus]

Okay, something has changed.

Before this PR, links (<a>) were specified to have the same color as main text. Now, since we've agreed to no longer specify main text color, I also stopped specifying link color. So they became blue (at least in Calibre).

Uh. I really wish there was a proper CSS rule to distinguish between color and monochrome devices.

[craigscott-crascit]

Initial results look good, I'm just waiting on feedback from one other device user.

That user has also confirmed it looks a lot better for them now on their device. They had previously provided a side-by-side comparison with other commercially produced content which highlighted the problem, but now the differences in darkness of the fonts are only slight.