Add theme switch

src/utilities/theme_handler.ts

@@ -0,0 +1,22 @@
+export function getTheme():string {
+    return localStorage.getItem("theme") ?? "dark";
+}
+
+export function setTheme(theme:string):void {
+    if (theme !== "dark" && theme !== "light") { theme = "dark"; }
+    localStorage.setItem("theme", theme);
+}
+
+export function flipTheme():void {
+    setTheme(getTheme() === "dark" ? "light" : "dark");
+}
+
+export function applyTheme(theme?:string):void {
+    if (!theme) { theme = getTheme(); }
+
+    switch (theme) {
+        case "dark":    document.documentElement.classList.add("dark"); break;
+        case "light":   document.documentElement.classList.remove("dark"); break;
+        default:        document.documentElement.classList.add("dark"); break;
+    }
+}