Darkened contribution calendar.
What's the issue:
Contribution calendar was still showing up with usual bright colours.
What's fixed:
I darkened the calendar with the css from Github Dark 1.
Screenshots:

I almost like this :-) I really do like contributions heatmap from v1, but the grid background is too dark. I found that if I used #364952 instead, the calendar looks really great ...
Another thought occurred to me: since this theme has a bit more colour in it than v1, what about adding more colour to the heatmap? Dunno ... might not be enough gradations for that.
Only modified css relevant to the calendar. I had to tweak/remove some errors to get it working with stylish for me, but I left those out of the commit.
I'm not sure about the cell background; it feels too close to the first contribution color, but the background in the first image feels too dark.
Something in these changes is causing the PR review comment to have a white border.

How do you feel about #03A9F4 for the fill? It's the link color (per CONTRIBUTING.md) and it looks pretty good to me.