diff --git a/components/pages/StyledApp.tsx b/components/pages/StyledApp.tsx
index c60a1454..d6a4b23a 100644
--- a/components/pages/StyledApp.tsx
+++ b/components/pages/StyledApp.tsx
@@ -2,6 +2,7 @@ import { SessionConsumer } from "contexts/session";
 import { ThemeProvider } from "styled-components";
 import GlobalStyle from "styles/GlobalStyle";
 import themes from "styles/themes";
+import { DEFAULT_THEME } from "utils/constants";
 
 type StyledAppProps = {
   children: React.ReactNode;
@@ -10,7 +11,7 @@ type StyledAppProps = {
 const StyledApp = ({ children }: StyledAppProps): JSX.Element => (
   <SessionConsumer>
     {({ themeName }) => (
-      <ThemeProvider theme={themes[themeName] || themes.defaultTheme}>
+      <ThemeProvider theme={themes[themeName] || themes[DEFAULT_THEME]}>
         <GlobalStyle />
         {children}
       </ThemeProvider>
diff --git a/tsconfig.json b/tsconfig.json
index daff9f6a..6cf42d4d 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -12,6 +12,8 @@
     "esModuleInterop": true,
     "module": "esnext",
     "moduleResolution": "node",
+    "noImplicitReturns": true,
+    "noPropertyAccessFromIndexSignature": true,
     "resolveJsonModule": true,
     "isolatedModules": true,
     "jsx": "preserve"
diff --git a/utils/constants.ts b/utils/constants.ts
index f53a0b2a..4adc3bd6 100644
--- a/utils/constants.ts
+++ b/utils/constants.ts
@@ -2,6 +2,8 @@ import type { Size } from "components/system/Window/RndWindow/useResizable";
 
 export const DEFAULT_LOCALE = "en";
 
+export const DEFAULT_THEME = "defaultTheme";
+
 export const DEFAULT_WINDOW_SIZE: Size = {
   height: 200,
   width: 250,