diff --git a/src/server/utils/build-themes-list.ts b/src/server/utils/build-themes-list.ts
index 73dc53bc..74b910c7 100644
--- a/src/server/utils/build-themes-list.ts
+++ b/src/server/utils/build-themes-list.ts
@@ -4,15 +4,20 @@ import { readdir } from "fs/promises";
 const extraThemesFolder =
   process.env["LEMMY_UI_EXTRA_THEMES_FOLDER"] || "./extra_themes";
 
-const themes = ["darkly", "darkly-red", "litely", "litely-red"];
+const themes: ReadonlyArray<string> = [
+  "darkly",
+  "darkly-red",
+  "litely",
+  "litely-red",
+];
 
-export async function buildThemeList(): Promise<string[]> {
+export async function buildThemeList(): Promise<ReadonlyArray<string>> {
   if (existsSync(extraThemesFolder)) {
     const dirThemes = await readdir(extraThemesFolder);
     const cssThemes = dirThemes
       .filter(d => d.endsWith(".css"))
       .map(d => d.replace(".css", ""));
-    themes.push(...cssThemes);
+    return themes.concat(cssThemes);
   }
   return themes;
 }