Change default dark theme colors similar to GitHub's dark theme

[tuurep]

We've considered that GitHub's dark theme colors make a more sensible default theme for Vivify, as the light theme mimics GitHub's light theme.

Origin story: before #17 Vivify only had Jannis's custom system-wide dark theme colors (which is still the default dark theme), and as I wanted to use a light theme, I copied the colors from GitHub as something that

looks as "normal" as possible

and we made that the default light theme.

There has been confusion about the dark theme not looking right: #290 (comment)

Plan:

1. Mimic a new default dark theme from GitHub's dark theme colors
2. Create a discussion about theming, where we preserve Jannis's theme (called Jellyfish) for those who like that theme, and encourage creating and sharing color themes in the thread

Note:

Vivify is not meant to sync with GitHub's markdown styling 100%, so I want to emphasize that the goal here is a sensible default set for colors, and to be easy to customize from there (both colors and CSS).

There are other tools with a goal to look exactly like GitHub, such as go-grip and gh-markdown-preview, better suited for that purpose.