Provide 8-color/3-bit fallback

[MithicSpirit]

Is your feature request related to a problem? Please describe. Using the catppuccin theme makes neovim terrible to use in a regular TTY, since everything is black and white.

Describe the solution you'd like I think that it would be helpful to provide an 8-color/3-bit fallback for such situations so that it's still usable, even if it doesn't look as good as the normal theme.

Describe alternatives you've considered I could also conditionally set up catppuccin or another theme, depending on my terminal's capabilities.