diff --git a/book.toml b/book.toml
index ca61a466..92f566be 100644
--- a/book.toml
+++ b/book.toml
@@ -9,7 +9,7 @@ title = "The Book of sbt"
 theme = "theme"
 default-theme = "light"
 additional-css = [
-  "src/reference/custom-202410.css",
+  "src/reference/custom-202509.css",
   "src/reference/mdbook-admonish.css",
   "theme/language-picker.css"
 ]
diff --git a/po/summary/getting-started/why-sbt-exists.ja.po b/po/summary/getting-started/why-sbt-exists.ja.po
index 6b8a24cf..506c59d6 100644
--- a/po/summary/getting-started/why-sbt-exists.ja.po
+++ b/po/summary/getting-started/why-sbt-exists.ja.po
@@ -2,7 +2,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: The Book of sbt\n"
 "POT-Creation-Date: 2025-09-15T01:45:08-04:00\n"
-"PO-Revision-Date: 2025-09-15 00:47-0400\n"
+"PO-Revision-Date: 2025-09-15 03:29-0400\n"
 "Last-Translator: Automatically generated\n"
 "Language-Team: none\n"
 "Language: ja\n"
@@ -135,6 +135,11 @@ msgid ""
 "Google's Bazel, Meta's Buck, and Apple's SwiftPM use DSL to define the "
 "subproject."
 msgstr ""
+"sbt はサブプロジェクトとタスクグラフを宣言するのに Scala 言語をベースとする "
+"build.sbt DSL を採用する。昨今では、YAML や XML といった設定形式の代わりに "
+"DSL を使っていることは sbt に限らない。Gradle、Google 由来の Bazel 、Meta の "
+"Buck、Apple の SwiftPM など多くのビルドツールが DSL を用いてサブプロジェクト"
+"を定義する。"
 
 #: src/reference/guide/why-sbt-exists.md:48
 msgid ""
@@ -142,6 +147,9 @@ msgid ""
 "and `libraryDependencies`, but it can scale as your need for the build "
 "system grows:"
 msgstr ""
+"`build.sbt` は、`scalaVersion` と `libraryDependencies` のみを宣言すればあた"
+"かも YAML ファイルのように始めることができるが、ビルドシステムへの要求が高度"
+"になってもスケールすることができる。"
 
 #: src/reference/guide/why-sbt-exists.md:51
 msgid ""
diff --git a/src/reference/custom-202509.css b/src/reference/custom-202509.css
new file mode 100644
index 00000000..84b4fa1b
--- /dev/null
+++ b/src/reference/custom-202509.css
@@ -0,0 +1,78 @@
+html:lang(en) {
+  font-family: "Lora", 'serif';
+}
+
+html:lang(zh-cn) {
+  font-family: "Lora", "Noto Serif", 'serif';
+}
+
+html:lang(ja) {
+  font-family: "Lora", "游明朝体", "Yu Mincho", YuMincho, "Noto Serif", serif;
+}
+
+h1 {
+  font-family: 'Linux Biolinum', "Noto Serif", 'serif';
+}
+
+h1:lang(en) {
+  font-family: 'Linux Biolinum', 'serif';
+}
+
+:root {
+  font-size: 80%;
+}
+
+pre {
+  border: solid 1px lightgray;
+  line-height: 1.8rem;
+  font-weight: 500;
+}
+
+code {
+  font-family: 'Inconsolata', monospace !important;
+}
+
+p code {
+  font-weight: 600;
+}
+
+li code {
+  font-weight: 600;
+}
+
+h1.menu-title:lang(en) {
+  font-family: "Lora",
+    'serif';
+  font-size: 1.5rem;
+}
+
+h1.menu-title:lang(ja) {
+  font-family: "Lora",
+    "游明朝体", "Yu Mincho", YuMincho,
+    "Noto Serif", 'serif';
+  font-size: 1.5rem;
+}
+
+h1.menu-title:lang(zh-cn) {
+  font-family: "Lora",
+    "Noto Serif", 'serif';
+  font-size: 1.5rem;
+}
+
+h2 {
+  font-family: 'Linux Biolinum',
+    "Noto Serif", 'serif';
+}
+
+h2:lang(en) {
+  font-family: 'Linux Biolinum', 'serif';
+}
+
+h3 {
+  font-family: 'Linux Biolinum',
+    "Noto Serif", 'serif';
+}
+
+h3:lang(en) {
+  font-family: 'Linux Biolinum', 'serif';
+}