Improve UI continuity behind panels (#1435)

* Add variable for island color

* Make islands semi-transparent

This preserves the notion of the infinite canvas and helps
maintain context, especially on smaller screens.

src/components/Island.css

@@ -1,6 +1,7 @@
 .Island {
   --padding: 0;
-  background-color: var(--bg-color-main);
+  background-color: var(--bg-color-island);
+  backdrop-filter: saturate(180%) blur(20px);
   box-shadow: var(--shadow-island);
   border-radius: var(--border-radius-m);
   padding: calc(var(--padding) * var(--space-factor));