Start adding CSS

static/by.css

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+html,body {
+    height: 100%;
+    margin: 0px;
+    padding: 0px;
+}
+
+body {
+    font-family: "Arial", "Helvetica", sans-serif;
+}
+
+.sidebar {
+    height: 100%;
+    width: 7em;
+    color: #fff;
+    background-color: #c00;
+    text-align: center;
+}
+
+.page {
+    margin: 2em;
+    width: 40em;
+    padding: 1em;
+    border: solid black 1px;
+}
+
+.board {
+    height: 100%;
+    display: flex;
+    flex-direction: row;
+}
+
+#root {
+    height: 100%;
+}