화면 mobile 버전으로 변경

client/src/App.js

@@ -11,7 +11,7 @@ import Err from "./pages/ErrorPage";
 function App() {
   return (
     <Router basename={process.env.PUBLIC_URL}>
-      <div className="mx-auto" style={{ width: "400px" }}>
+      <div id="box" className="mx-sm-auto">
         <Switch>
           <Route exact path="/" component={Err} />
           <Route path="/signup" component={SignupPage} />

client/src/scss/custom.scss

 @import "../../node_modules/bootstrap/scss/functions";
 @import "~bootstrap/scss/variables";
+@import "~bootstrap/scss/mixins";
 
 // @import '~bootstrap/scss/bootstrap.scss';
 // @import '~bootstrap-icons/font/bootstrap-icons'; 
@@ -41,4 +42,10 @@
 
 h1, h2, h3, h4, h5, h6 {
     font-family: "Plex-Bold";
+}
+
+@include media-breakpoint-up(sm) {
+    #box {
+        width: 400px;
+    }
 }
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