Fix theme shop themes not applying as a base

- [#217] fixed theme shop themes not applying as a base
- added `themes` state path to store themes locally at

src/redux/reducers/themes.ts

@@ -0,0 +1,33 @@
+import { Theme } from "../../context/Theme";
+
+import { ThemeMetadata } from "../../pages/settings/panes/ThemeShop";
+
+export interface StoredTheme {
+    slug: string;
+    meta: ThemeMetadata;
+    theme: Theme;
+}
+
+export type Themes = Record<string, StoredTheme>;
+
+export type ThemesAction =
+    | { type: undefined }
+    | { type: "THEMES_SET_THEME"; theme: StoredTheme }
+    | { type: "THEMES_REMOVE_THEME"; slug: string }
+    | { type: "RESET" };
+
+export function themes(state: Themes = {}, action: ThemesAction) {
+    switch (action.type) {
+        case "THEMES_SET_THEME":
+            return {
+                ...state,
+                [action.theme.slug]: action.theme,
+            };
+        case "THEMES_REMOVE_THEME":
+            return { ...state, [action.slug]: null };
+        case "RESET":
+            return {};
+        default:
+            return state;
+    }
+}