css: Document replacements for color stuff

Document how named colors and non-standard color expressions
can be replaced.

docs/reference/gtk/migrating-4to5.md

@@ -73,6 +73,103 @@ GTK 5 will no longer provide this functionality. The recommendations
 is to use a global stylesheet (i.e. gtk_style_context_add_provider_for_display())
 and rely on style classes to make your CSS apply only where desired.
 
+## Non-standard CSS extensions are going away
+
+GTK's CSS machinery has a some non-standard extensions around colors:
+named colors with \@define-color and color functions: lighter(), darker(),
+shade(), alpha(), mix().
+
+GTK now implements equivalent functionality from the CSS specs.
+
+### \@define-color is going away
+
+\@define-color should be replaced by custom properties in the :root scope.
+
+Instead of
+
+```
+@define-color fg_color #2e3436
+
+...
+
+box {
+  color: @fg_color;
+}
+```
+
+use
+
+```
+:root {
+  --fg-color: #2e3436;
+}
+
+...
+
+box {
+  color: var(--fg-color);
+}
+```
+
+For more information about custom CSS properties and variables, see the
+[CSS Custom Properties for Cascading Variables](https://www.w3.org/TR/css-variables-1/)
+spec.
+
+### Color expressions are going away
+
+The color functions can all be replaced by combinations of calc() and color-mix().
+
+ligher(c) and darker(c) are just shade(c, 1.3) or shade(c, 0.7), respectively, and
+thus can be handled the same way as shade in the examples below.
+
+Replace
+
+```
+a {
+  color: mix(red, green, 0.8);
+}
+
+b {
+  color: alpha(black, 0.5);
+}
+
+c {
+  color: shade(red, 1.3);
+}
+
+d {
+  color: shade(red, 0.7);
+}
+```
+
+with
+
+```
+a {
+  color: color-mix(in srgb, red, green 80%);
+}
+
+b {
+  color: color-mix(in srgb, black, transparent 50%);
+}
+
+c {
+  color: color-mix(in hsl, red, white 30%);
+}
+
+d {
+  color: color-mix(in hsl, red, black 30%);
+}
+```
+
+Variations of these replacements are possible.
+
+Note that GTK has historically computed mix() and shade() values in the SRGB and HSL
+colorspaces, but using OKLAB instead might yield slightly better results.
+
+For more information about color-mix(), see the
+[CSS Color](https://drafts.csswg.org/css-color-5) spec.
+
 ## Chooser interfaces are going away
 
 The GtkColorChooser, GtkFontChooser, GtkFileChooser and GtkAppChooser