Make the footer stick to bottom

Currently, the footer doesn't stick to the bottom of the viewport when the page content is small. This fixes the issue by making the <main> element fill the remaining space

assets/css/screen.css

@@ -68,6 +68,10 @@ production stylesheet in assets/built/screen.css
     min-height: 100vh;
 }
 
+.viewport > main {
+    flex-grow: 1;
+}
+
 /* Full width page blocks */
 .outer {
     position: relative;