diff --git a/packages/@vivliostyle/theme-base/css/lib/prism/theme-okaidia.css b/packages/@vivliostyle/theme-base/css/lib/prism/theme-okaidia.css
index 718d2d9..6644187 100644
--- a/packages/@vivliostyle/theme-base/css/lib/prism/theme-okaidia.css
+++ b/packages/@vivliostyle/theme-base/css/lib/prism/theme-okaidia.css
@@ -30,7 +30,6 @@
   --vs-prism--color-operator: #f8f8f2;
   --vs-prism--color-entity: #f8f8f2;
   --vs-prism--color-url: #f8f8f2;
-  --vs-prism--color-string: #f8f8f2;
   --vs-prism--color-variable: #f8f8f2;
 
   --vs-prism--color-atrule: #e6db74;
diff --git a/packages/@vivliostyle/theme-base/css/lib/prism/theme-prism.css b/packages/@vivliostyle/theme-base/css/lib/prism/theme-prism.css
index fb5c609..47661d7 100644
--- a/packages/@vivliostyle/theme-base/css/lib/prism/theme-prism.css
+++ b/packages/@vivliostyle/theme-base/css/lib/prism/theme-prism.css
@@ -29,7 +29,6 @@
   --vs-prism--color-operator: #9a6e3a;
   --vs-prism--color-entity: #9a6e3a;
   --vs-prism--color-url: #9a6e3a;
-  --vs-prism--color-string: #9a6e3a;
 
   --vs-prism--color-atrule: #07a;
   --vs-prism--color-attr-value: #07a;