diff --git a/source/ts/single-page-application.ts b/source/ts/single-page-application.ts
index 0ed542d..1ddba76 100644
--- a/source/ts/single-page-application.ts
+++ b/source/ts/single-page-application.ts
@@ -9,11 +9,11 @@ import {Router} from 'preact-router';
 import HomePage from './pages/home.js';
 import NotFoundPage from './pages/not-found.js';
 import ReleasePage from './pages/release.js';
-import {themeContext, themes} from './utilities/themes.js';
+import {getThemeByCssClass, themeContext} from './utilities/themes.js';
 
-const savedTheme = window.localStorage.getItem('theme');
-const activeTheme =
-  themes.find((theme) => theme.cssClass === savedTheme) ?? themes[0];
+const activeTheme = getThemeByCssClass(
+  window.localStorage.getItem('theme') ?? '',
+);
 
 document.body.classList.value = activeTheme.cssClass;
 
diff --git a/source/ts/utilities/themes.ts b/source/ts/utilities/themes.ts
index 6eec3bb..0a750f0 100644
--- a/source/ts/utilities/themes.ts
+++ b/source/ts/utilities/themes.ts
@@ -17,3 +17,12 @@ export const themes: Theme[] = [
 ];
 
 export const themeContext = createContext<Theme>(themes[0]);
+
+export function getThemeByCssClass(cssClass: string): Theme {
+  return themes.find((theme) => theme.cssClass === cssClass) ?? themes[0];
+}
+
+export function setTheme(theme: Theme): void {
+  document.body.classList.value = theme.cssClass;
+  window.localStorage.setItem('theme', theme.cssClass);
+}