chore: interfaces for theme store (#584)

# Motivation

Improve the types of the theme store by declaring interfaces as we
commonly do nowadays.

---------

Co-authored-by: Yusef Habib <[email protected]>

Author: peterpeterparker

src/lib/stores/theme.store.ts

@@ -1,11 +1,17 @@
 import type { Theme } from "$lib/types/theme";
 import { applyTheme, initTheme } from "$lib/utils/theme.utils";
-import { writable } from "svelte/store";
+import { writable, type Readable } from "svelte/store";
 
-const initialTheme: Theme | undefined = initTheme();
+export type ThemeStoreData = Theme | undefined;
 
-export const initThemeStore = () => {
-  const { subscribe, set } = writable<Theme | undefined>(initialTheme);
+export interface ThemeStore extends Readable<ThemeStoreData> {
+  select: (theme: Theme) => void;
+}
+
+const initialTheme: ThemeStoreData = initTheme();
+
+export const initThemeStore = (): ThemeStore => {
+  const { subscribe, set } = writable<ThemeStoreData>(initialTheme);
 
   return {
     subscribe,