Gracefully degrade theme colors on terminals with low color depth

[shipmints]

When a deep-palette theme like modus-themes-vivendi is enabled on a terminal with fewer colors than needed to fully render the theme, it can become nearly unusable.

I do not know precisely how Emacs maps color requests from a 256(+) color palette to 16, and if there is a method to leverage, we should have a function in modus-themes (or per https://debbugs.gnu.org/cgi/bugreport.cgi?bug=79696#68 some upstreamed function) that remaps colors to an actually visible, reduced color depth.

The trigger for an implementation could be based on the functions display-color-p and display-color-cells (see https://www.gnu.org/software/emacs/manual/html_node/elisp/Display-Feature-Testing.html).

[protesilaos]

Hello @shipmints!

The assumption I have is that in that environment the user is better off not loading a theme anyway.

That granted, we now have ,c for the display specification, which is ((class color) (min-colors 256)). We could have t as the fallback, though then we need some non-trivial way to get to a style that is decent. For some cases this is obvious, like using :inverse-video, but for many things it is not clear what to do. Like, how would a Flymake underline error/warning/note look? Can it colourise underlines? Should it use a background instead? It will get really tricky like that.

As an alternative, it would make more sense to have a new pair of themes that are designed specifically for this case (assuming Emacs does not do something right by default).

[shipmints]

I think skipping loading the theme at all is a nice, easy solution, perhaps with a message that indicates the reason so people can understand when this case occurs.

We could keep the graceful degradation idea for the future and especially if people want to upstream your functionality that would be nice to noodle over to make theming more robust.

[shipmints]

I've changed my modus-themes use-package stanza as follows:

(use-feature modus-themes
  :unless (or noninteractive
              (< (display-color-cells) 256))

Perhaps this can be a documentation update for the time being.