diff --git a/server/plugins/packages/themes.ts b/server/plugins/packages/themes.ts
index 0594786f..67006a4f 100644
--- a/server/plugins/packages/themes.ts
+++ b/server/plugins/packages/themes.ts
@@ -85,7 +85,7 @@ function makePackageThemeObject(
 	return {
 		displayName: module.name || moduleName,
 		filename: path.join(modulePath, module.css),
-		name: moduleName,
+		name: encodeURIComponent(moduleName),
 		themeColor: themeColor,
 	};
 }
diff --git a/server/server.ts b/server/server.ts
index ab8d2419..29f696d3 100644
--- a/server/server.ts
+++ b/server/server.ts
@@ -107,7 +107,7 @@ export default async function (
 	// handler. Remember this if you make changes to this function, serving of
 	// local themes will not get those changes.
 	app.get("/themes/:theme.css", (req, res) => {
-		const themeName = req.params.theme;
+		const themeName = encodeURIComponent(req.params.theme);
 		const theme = themes.getByName(themeName);
 
 		if (theme === undefined || theme.filename === undefined) {