Themes and/or "dark mode"

[AndrewRadev]

Right now, there's only one CSS used, github's default one, which I literally ripped out of the site. It would be nice to allow changing the CSS to the dark-mode version and possibly others, like crates.io's markdown rendering. So, --theme github-light, --theme github-dark, --theme crates.io.

Alternatively, it would be plugged into the file config -- that would make it easier to pick just one, but harder to switch. Could be that "both" would be a good option for this particular setting.

[cheap-glitch]

It would be nice to also support images bound to a specific theme.

Alternatively, it would be plugged into the file config -- that would make it easier to pick just one, but harder to switch.

Now that custom key mappings are implemented, adding a ToggleTheme action would alleviate the issue. There could even be PreviousTheme and NextTheme actions to allow cycling through every variant of the light and dark modes.