Fix theme handling and auto theme

This commit fixes the client side theme selection being overwritten by the backend and adds a selectable auto theming option.

ui/src/app/theme.ts

@@ -0,0 +1,26 @@
+import { IconDefinition } from "@fortawesome/fontawesome-svg-core";
+import { faCircleHalfStroke, faMoon, faSun  } from "@fortawesome/free-solid-svg-icons";
+
+export interface Theme {
+  id: string;
+  displayName: string;
+  icon: IconDefinition;
+}
+
+export const Themes: Theme[] = [
+  {
+    id: 'light',
+    displayName: 'Light',
+    icon: faSun,
+  },
+  {
+    id: 'dark',
+    displayName: 'Dark',
+    icon: faMoon,
+  },
+  {
+    id: 'auto',
+    displayName: 'Auto',
+    icon: faCircleHalfStroke,
+  },
+];