diff --git a/app/javascript/styles/mastodon-auto.scss b/app/javascript/styles/mastodon-auto.scss
new file mode 100644
index 000000000..ccb5d24eb
--- /dev/null
+++ b/app/javascript/styles/mastodon-auto.scss
@@ -0,0 +1,2 @@
+@use 'application';
+@media (prefers-color-scheme: light) { @import 'mastodon-light'; }
\ No newline at end of file
diff --git a/config/locales/en.yml b/config/locales/en.yml
index e8393484d..d4b77535e 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -1242,6 +1242,7 @@ en:
     contrast: Mastodon (High contrast)
     default: Mastodon (Dark)
     mastodon-light: Mastodon (Light)
+    mastodon-auto: Mastodon (Auto)
     # additional theme
     material-v1-dark: Material v1 (Dark)
     material-v1-light: Material v1 (Light)
diff --git a/config/locales/ja.yml b/config/locales/ja.yml
index 9c7aeaa68..699902665 100644
--- a/config/locales/ja.yml
+++ b/config/locales/ja.yml
@@ -1224,6 +1224,7 @@ ja:
     contrast: Mastodon (ハイコントラスト)
     default: Mastodon (ダーク)
     mastodon-light: Mastodon (ライト)
+    mastodon-auto: Mastodon (自動)
     # 追加テーマ
     material-v1-dark: Material v1 (ダーク)
     material-v1-light: Material v1 (ライト)
diff --git a/config/themes.yml b/config/themes.yml
index 17945f652..5f6a22a6e 100644
--- a/config/themes.yml
+++ b/config/themes.yml
@@ -1,6 +1,7 @@
 default: styles/application.scss
 contrast: styles/contrast.scss
 mastodon-light: styles/mastodon-light.scss
+mastodon-auto: styles/mastodon-auto.scss
 
 # 追加テーマ
 material-v1-dark: styles/material-v1-dark.scss