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.

1 files changed, 11 insertions, 0 deletions

diff --git a/emcclib.js b/emcclib.js
index 3824293..814ff56 100644
--- a/emcclib.js
+++ b/emcclib.js
@@ -144,6 +144,17 @@ mergeInto(LibraryManager.library, {
     },
 
     /*
+     * 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);
      *
      * Return the current date, in milliseconds since the epoch