[EDIT]スマホ画面のデザインに統一しました

src/main/resources/templates/header.html

@@ -4,12 +4,13 @@
 <meta charset="UTF-8" name="viewport" content="width=device-width, initial-scale=1">
 <title>ヘッダー</title>
 <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.7/dist/css/bootstrap.min.css" rel="stylesheet">
+<link rel="stylesheet" th:href="@{/css/style.css}">
 </head>
 <body>
 	
 	<header th:fragment="header" class="fixed-top">
 
-		<nav class="navbar" style="background-color: #e3f2fd;" data-bs-theme="light">
+		<nav class="navbar app-header" style="background-color: #e3f2fd;" data-bs-theme="light">
 		  <div class="container-fluid">
 		    <a th:href="@{/menu}" class="navbar-brand fw-bold">Monegement</a>
 			<div class="d-flex gap-2">