Updated.

book.json

@@ -57,7 +57,8 @@
       "tag": "b"
     },
     "add-js-css": {
-      "css": [ "./custom-style.css" ]
+      "css": [ "./custom_style.css" ],
+      "js": [ "./custom_script.js" ]
     },
     "navigation": {
       "navigatorList": [

custom_script.js

@@ -0,0 +1,5 @@
+var honkit_link = $('a[class=gitbook-link]');
+
+var text = honkit_link.text();
+honkit_link.parent().html(`<span style="align: center;">${text}</span>`);
+

custom_style.css

@@ -0,0 +1,53 @@
+div.book-header {
+  position: fixed;
+}
+
+::-webkit-scrollbar {
+  width: 5px;
+}
+
+::-webkit-scrollbar-track {
+  background-color: #e4e4e4;
+  border-radius: 100px;
+}
+
+::-webkit-scrollbar-thumb {
+  box-shadow: inset 0 0 6px rgba(0, 0, 0, 0.3);
+}
+
+li.nav-item>a {
+  text-decoration: none;
+}
+
+i.fa {
+  font-weight: bold;
+  color: #5f5f5f;
+}
+
+.page-toc-menu {
+  background-color: #555555 !important;
+  color: #cccccc !important;
+  border-radius: 10px;
+}
+
+@media only screen and (min-width: 1024px) {
+  div.book-header {
+    top: 30px;
+  }
+  .nav-list .nav-item a {
+    font-size: small !important;
+  }
+  .nav-list .nav-item a:hover {
+    color: #f59542 !important;
+  }
+}
+
+@media only screen and (max-width: 1024px) {
+  ul.nav-list {
+    display: none;
+  }
+
+  div.book-headr {
+    top: 5px;
+  }
+}