Fix page format and change document title

index.html

@@ -3,7 +3,7 @@
 <head>
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
-    <title>Document</title>
+    <title>Calculator</title>
     <link rel="stylesheet" href="style.css">
 </head>
 <body>

style.css

@@ -1,9 +1,11 @@
+
 body {
+    display: flex;
     text-align: center;
     flex-direction: column;
-    min-height: 100vh;
+    height: 100vh;
     margin: 0;
-    background-color: #111;
+    background-color: rgb(9, 73, 9);
     color: #eee;
 }
 
@@ -12,12 +14,11 @@ header {
 }
 
 main {
+    flex: 1;
     display: flex;
     flex-direction: column;
     justify-content: center;
     align-items: center;
-    flex: 1;
-
 }
 
 .calculator {