fix for seamonkey

doesn't support image-set()

base.css

@@ -16,6 +16,18 @@
   --link-color: hsl(196deg, 100%, 85%);
   --hover-color: hsl(50deg, 100%, 82%);
 
+  /* for seamonkey ☺ */
+  background:
+    url(media/bg.png)
+      top center fixed,
+    linear-gradient(135deg,
+        hsl(42deg,  92%,  70%),
+        hsl(348deg, 92%,  70%),
+        hsl(334deg, 100%, 80%),
+        hsl(234deg, 100%, 76%),
+        hsl(195deg, 100%, 67%),
+        hsl(155deg, 70%,  62%)
+      ) fixed;
   background:
     -webkit-image-set(url(media/bg.png)    1x,
                       url(media/bg_2x.png) 2x)