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));

src/css/theme.scss

@@ -2,6 +2,7 @@
   --text-color-primary: #343a40;
   --bg-color-main: #fff;
   --shadow-island: 0 1px 5px rgba(0, 0, 0, 0.15);
+  --bg-color-island: rgba(255, 255, 255, 0.7);
   --border-radius-m: 4px;
   --space-factor: 0.25rem;
 }