Add option to select system theme

src/components/context/ThemeModeContext.tsx

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+import React from 'react';
+
+export enum ThemeMode {
+    SYSTEM = 'system',
+    DARK = 'dark',
+    LIGHT = 'light',
+}
+
+type ContextType = {
+    themeMode: ThemeMode;
+    setThemeMode: React.Dispatch<React.SetStateAction<ThemeMode>>;
+};
+
+export const ThemeModeContext = React.createContext<ContextType>({
+    themeMode: ThemeMode.SYSTEM,
+    setThemeMode: (): void => {},
+});