Use display-color-cells to enable 24-bit color terminal support (#88)

In order to fix "unable to load color x" errors when running Emacs in
daemon-mode, the defined theme colors are now tested against the
`display-color-cells` "display feature testing" function [1] to support
24-bit color terminals with a fallback to the `display-graphic-p`
function.

This is a first step to fix the problems described in GH-59.

Related to GH-59

nord-theme.el

@@ -93,39 +93,43 @@
   Please see https://github.com/arcticicestudio/nord-emacs/issues/73 for more details."
   "0.4.0")
 
+(defun nord-display-truecolor-or-graphic-p ()
+  "Returns whether the display can display nord colors"
+  (or (= (display-color-cells) 16777216) (display-graphic-p)))
+
 ;;;; Color Constants
 (let ((class '((class color) (min-colors 89)))
-  (nord0 (if (display-graphic-p) "#2E3440" nil))
-  (nord1 (if (display-graphic-p) "#3B4252" "black"))
-  (nord2 (if (display-graphic-p) "#434C5E" "#434C5E"))
-  (nord3 (if (display-graphic-p) "#4C566A" "brightblack"))
-  (nord4 (if (display-graphic-p) "#D8DEE9" "#D8DEE9"))
-  (nord5 (if (display-graphic-p) "#E5E9F0" "white"))
-  (nord6 (if (display-graphic-p) "#ECEFF4" "brightwhite"))
-  (nord7 (if (display-graphic-p) "#8FBCBB" "cyan"))
-  (nord8 (if (display-graphic-p) "#88C0D0" "brightcyan"))
-  (nord9 (if (display-graphic-p) "#81A1C1" "blue"))
-  (nord10 (if (display-graphic-p) "#5E81AC" "brightblue"))
-  (nord11 (if (display-graphic-p) "#BF616A" "red"))
-  (nord12 (if (display-graphic-p) "#D08770" "brightyellow"))
-  (nord13 (if (display-graphic-p) "#EBCB8B" "yellow"))
-  (nord14 (if (display-graphic-p) "#A3BE8C" "green"))
-  (nord15 (if (display-graphic-p) "#B48EAD" "magenta"))
-  (nord-annotation (if (display-graphic-p) "#D08770" "brightyellow"))
-  (nord-attribute (if (display-graphic-p) "#8FBCBB" "cyan"))
-  (nord-class (if (display-graphic-p) "#8FBCBB" "cyan"))
-  (nord-comment (if (display-graphic-p) (nord-theme--brightened-comment-color nord-comment-brightness) "brightblack"))
-  (nord-escape (if (display-graphic-p) "#D08770" "brightyellow"))
-  (nord-method (if (display-graphic-p) "#88C0D0" "brightcyan"))
-  (nord-keyword (if (display-graphic-p) "#81A1C1" "blue"))
-  (nord-numeric (if (display-graphic-p) "#B48EAD" "magenta"))
-  (nord-operator (if (display-graphic-p) "#81A1C1" "blue"))
-  (nord-preprocessor (if (display-graphic-p) "#5E81AC" "brightblue"))
-  (nord-punctuation (if (display-graphic-p) "#D8DEE9" "#D8DEE9"))
-  (nord-regexp (if (display-graphic-p) "#EBCB8B" "yellow"))
-  (nord-string (if (display-graphic-p) "#A3BE8C" "green"))
-  (nord-tag (if (display-graphic-p) "#81A1C1" "blue"))
-  (nord-variable (if (display-graphic-p) "#D8DEE9" "#D8DEE9"))
+  (nord0 (if (nord-display-truecolor-or-graphic-p) "#2E3440" nil))
+  (nord1 (if (nord-display-truecolor-or-graphic-p) "#3B4252" "black"))
+  (nord2 (if (nord-display-truecolor-or-graphic-p) "#434C5E" "#434C5E"))
+  (nord3 (if (nord-display-truecolor-or-graphic-p) "#4C566A" "brightblack"))
+  (nord4 (if (nord-display-truecolor-or-graphic-p) "#D8DEE9" "#D8DEE9"))
+  (nord5 (if (nord-display-truecolor-or-graphic-p) "#E5E9F0" "white"))
+  (nord6 (if (nord-display-truecolor-or-graphic-p) "#ECEFF4" "brightwhite"))
+  (nord7 (if (nord-display-truecolor-or-graphic-p) "#8FBCBB" "cyan"))
+  (nord8 (if (nord-display-truecolor-or-graphic-p) "#88C0D0" "brightcyan"))
+  (nord9 (if (nord-display-truecolor-or-graphic-p) "#81A1C1" "blue"))
+  (nord10 (if (nord-display-truecolor-or-graphic-p) "#5E81AC" "brightblue"))
+  (nord11 (if (nord-display-truecolor-or-graphic-p) "#BF616A" "red"))
+  (nord12 (if (nord-display-truecolor-or-graphic-p) "#D08770" "brightyellow"))
+  (nord13 (if (nord-display-truecolor-or-graphic-p) "#EBCB8B" "yellow"))
+  (nord14 (if (nord-display-truecolor-or-graphic-p) "#A3BE8C" "green"))
+  (nord15 (if (nord-display-truecolor-or-graphic-p) "#B48EAD" "magenta"))
+  (nord-annotation (if (nord-display-truecolor-or-graphic-p) "#D08770" "brightyellow"))
+  (nord-attribute (if (nord-display-truecolor-or-graphic-p) "#8FBCBB" "cyan"))
+  (nord-class (if (nord-display-truecolor-or-graphic-p) "#8FBCBB" "cyan"))
+  (nord-comment (if (nord-display-truecolor-or-graphic-p) (nord-theme--brightened-comment-color nord-comment-brightness) "brightblack"))
+  (nord-escape (if (nord-display-truecolor-or-graphic-p) "#D08770" "brightyellow"))
+  (nord-method (if (nord-display-truecolor-or-graphic-p) "#88C0D0" "brightcyan"))
+  (nord-keyword (if (nord-display-truecolor-or-graphic-p) "#81A1C1" "blue"))
+  (nord-numeric (if (nord-display-truecolor-or-graphic-p) "#B48EAD" "magenta"))
+  (nord-operator (if (nord-display-truecolor-or-graphic-p) "#81A1C1" "blue"))
+  (nord-preprocessor (if (nord-display-truecolor-or-graphic-p) "#5E81AC" "brightblue"))
+  (nord-punctuation (if (nord-display-truecolor-or-graphic-p) "#D8DEE9" "#D8DEE9"))
+  (nord-regexp (if (nord-display-truecolor-or-graphic-p) "#EBCB8B" "yellow"))
+  (nord-string (if (nord-display-truecolor-or-graphic-p) "#A3BE8C" "green"))
+  (nord-tag (if (nord-display-truecolor-or-graphic-p) "#81A1C1" "blue"))
+  (nord-variable (if (nord-display-truecolor-or-graphic-p) "#D8DEE9" "#D8DEE9"))
   (nord-region-highlight-foreground (if (or
     (string= nord-region-highlight "frost")
     (string= nord-region-highlight "snowstorm")) "#2E3440" nil))