Adwaita: add public color for text views

- define @text_view_bg to complement @theme_text_color for high
  contrast legible text views.

Fixes https://gitlab.gnome.org/GNOME/vte/-/issues/226

gtk/theme/Adwaita/_colors-public.scss

@@ -112,5 +112,8 @@ read if you used those and something break with a version upgrade you're on your
 
 //FIXME this is really an API
 
+/* content view background such as thumbnails view in Photos or Boxes */
 @define-color content_view_bg #{"" + $base_color};
 
+/* Very contrasty background for text views (@theme_text_color foreground) */
+@define-color text_view_bg #{"" + if($variant == 'light', $base_color, darken($base_color,6%))};