Brighten org headline levels 7 and 8.

[holocronweaver]

The previous deepest org headline colors were so dark they were difficult to see against the background, especially for those with moderate colorblindness.

This change also mildly mixes up the org headline color palette for a little more variety.

Before:

After:

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• [X] The commits are consistent with our contribution guidelines
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Thanks!

[bbatsov]

I like your proposal. Thanks!