Refactor user preferences (#4667)

# Description of Changes
Refactor user preferences to all be in one service and all stored in
localStorage instead of indexeddb. This allows simpler & quicker
accessing of them, and ensures that they're all neatly stored in one
consistent place instead of spread out over local storage.

frontend/src/constants/theme.ts

@@ -0,0 +1,8 @@
+// Theme constants and utilities
+
+export type ThemeMode = 'light' | 'dark' | 'rainbow';
+
+// Detect OS theme preference
+export function getSystemTheme(): 'light' | 'dark' {
+  return window?.matchMedia?.('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+}