Allow theme switching without local storage

Protect against a null `window.localStorage` result which can happen
if the user has disabled support.

Closes gh-50

Author: philwebb

src/main/js/setup/switchtheme.js

@@ -46,22 +46,30 @@
   }
 
   function isInitialThemeDark() {
-    return (
-      localStorage.getItem("theme") === "dark" || prefersDarkColorScheme.matches
-    );
+    return loadTheme() === "dark" || prefersDarkColorScheme.matches;
   }
 
   function onThemeChange() {
     if (this.checked) {
       delay(function () {
         htmlElement.classList.add("dark-theme");
       }, 100);
-      localStorage.setItem("theme", "dark");
+      saveTheme("dark");
     } else {
       delay(function () {
         htmlElement.classList.remove("dark-theme");
       }, 100);
-      localStorage.setItem("theme", "light");
+      saveTheme("light");
+    }
+  }
+
+  function saveTheme(theme) {
+    if (localStorage) {
+      localStorage.setItem("theme", theme);
     }
   }
+
+  function loadTheme() {
+    return localStorage !== null ? localStorage.getItem("theme") : null;
+  }
 })();