add theme selector functionality

scripts/preferences/changeTheme.js

@@ -0,0 +1,5 @@
+document.getElementById("theme-selector").value = localStorage.getItem("text-startpage:theme")
+
+document.getElementById("theme-selector").addEventListener("change", () => {
+    localStorage.setItem("text-startpage:theme", document.getElementById("theme-selector").value)
+})