🚀 Better default colors for dark terminal backgrounds

I think the default colors for dark terminal backgrounds could be improved. Does anyone have any thoughts about this? Do you have some dark mode colors that you're happy with and would you be willing to share your config? cc @ulwlu @kevinhwang91 @torarnv @infokiller @Kr1ss-XD @clnoll (please add any others who might have ideas here).

It might also be nice to have a sort of gallery of example configs and screenshots to give people some ideas for configuring and to show what's possible. These could be distributed in the delta repo as named features in a config file, which people could `include`, something like

```gitconfig
[include]
path = ~/src/delta/delta/themes.gitconfig
```

**EDIT**: ☝️ that exists now; see https://github.com/dandavison/delta#custom-color-themes

Start with the existing Custom Color Themes section in the README and review the discussion and configuration examples in this issue. Determine whether a concrete proposal for better dark-background defaults remains after the documented theme support; done would require an agreed color scheme and updated default or example configuration.

Written by the indexing model from the issue text.