js: Put the puzzle background colour in a CSS variable

It's sometimes useful to be able to have an HTML element that visually
forms an extension of the puzzle's border.  By putting the puzzle's
colour 0 (which we assume to be its background) into a CSS variable,
such elements can do something like "background-color:
var(--puzzle-background)" to get that effect even if the puzzle uses a
non-default background colour.

emcclib.js

@@ -143,6 +143,17 @@ mergeInto(LibraryManager.library, {
         menuform.elements["preset"].value = n;
     },
 
+    /*
+     * void js_set_background_colour(const char *bg);
+     *
+     * Record the puzzle background colour in a CSS variable so
+     * the style sheet can use it if it wants.
+     */
+    js_set_background_colour: function(bgptr) {
+        document.documentElement.style.setProperty("--puzzle-background",
+                                                   UTF8ToString(bgptr));
+    },
+
     /*
      * void js_get_date_64(unsigned *p);
      *