Make Themes color-theme-aware for fish 4.3

[simono]

fish 4.3 added support for switching between dark and light variants of a theme.

We could e.g. add Rosé Pine Dawn as a light variant to Rosé Pine Moon and Rosé Pine. This way you just set Rosé Pine Moon or Rosé Pine as theme and it automatically switches to Rosé Pine Dawn if you have a light terminal background.
Or we add separate variants.

Is this something that you'd accept as a PR @mvllow?

[mvllow]

I wouldn't mind having two additional files for these new versions. Depending how the file looks, we could maybe add a step in the build script to generate these new versions from the existing ones unless you have something else in mind :)

Edit: I personally love auto switching but I also know many people who do not, so leaving them separate would be preferred unless there's an env variable or something that fish reads to enable/disable this feature

[simono]

Alright, implemented that in #6 @mvllow 😄