diff --git a/packages/docusaurus-theme-classic/src/theme/hooks/useTheme.ts b/packages/docusaurus-theme-classic/src/theme/hooks/useTheme.ts
index b2a24c356c39..05a8fcff1cdf 100644
--- a/packages/docusaurus-theme-classic/src/theme/hooks/useTheme.ts
+++ b/packages/docusaurus-theme-classic/src/theme/hooks/useTheme.ts
@@ -14,21 +14,23 @@ import {useThemeConfig} from '@docusaurus/theme-common';
 const themes = {
   light: 'light',
   dark: 'dark',
-};
+} as const;
+
+type Themes = typeof themes[keyof typeof themes];
 
 // Ensure to always return a valid theme even if input is invalid
-const coerceToTheme = (theme) => {
+const coerceToTheme = (theme?: string | null): Themes => {
   return theme === themes.dark ? themes.dark : themes.light;
 };
 
-const getInitialTheme = (defaultMode) => {
+const getInitialTheme = (defaultMode: Themes | undefined): Themes => {
   if (!ExecutionEnvironment.canUseDOM) {
     return coerceToTheme(defaultMode);
   }
   return coerceToTheme(document.documentElement.getAttribute('data-theme'));
 };
 
-const storeTheme = (newTheme) => {
+const storeTheme = (newTheme: Themes) => {
   try {
     localStorage.setItem('theme', coerceToTheme(newTheme));
   } catch (err) {