diff --git a/eslint.config.mjs b/eslint.config.mjs
index 34d139d90..3d4b1229f 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -116,6 +116,7 @@ export default [
             "pdfjs-lib",
             "pdfjs-web",
             "web",
+            "@csstools/postcss-light-dark-function",
             "fluent-bundle",
             "fluent-dom",
             "postcss-dir-pseudo-class",
diff --git a/package-lock.json b/package-lock.json
index f9ebedb1a..ea66599ee 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10,7 +10,7 @@
         "@babel/core": "^7.29.0",
         "@babel/preset-env": "^7.29.0",
         "@babel/runtime": "^7.28.6",
-        "@csstools/postcss-light-dark-function": "^2.0.11",
+        "@csstools/postcss-light-dark-function": "^3.0.0",
         "@fluent/bundle": "^0.19.1",
         "@fluent/dom": "^0.10.2",
         "@metalsmith/layouts": "^3.0.0",
@@ -1667,9 +1667,9 @@
       }
     },
     "node_modules/@csstools/css-parser-algorithms": {
-      "version": "3.0.5",
-      "resolved": "https://registry.npmjs.org/@csstools/css-parser-algorithms/-/css-parser-algorithms-3.0.5.tgz",
-      "integrity": "sha512-DaDeUkXZKjdGhgYaHNJTV9pV7Y9B3b644jCLs9Upc3VeNGg6LWARAT6O+Q+/COo+2gg/bM5rhpMAtf70WqfBdQ==",
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/css-parser-algorithms/-/css-parser-algorithms-4.0.0.tgz",
+      "integrity": "sha512-+B87qS7fIG3L5h3qwJ/IFbjoVoOe/bpOdh9hAjXbvx0o8ImEmUsGXN0inFOnk2ChCFgqkkGFQ+TpM5rbhkKe4w==",
       "dev": true,
       "funding": [
         {
@@ -1683,10 +1683,10 @@
       ],
       "license": "MIT",
       "engines": {
-        "node": ">=18"
+        "node": ">=20.19.0"
       },
       "peerDependencies": {
-        "@csstools/css-tokenizer": "^3.0.4"
+        "@csstools/css-tokenizer": "^4.0.0"
       }
     },
     "node_modules/@csstools/css-syntax-patches-for-csstree": {
@@ -1707,9 +1707,9 @@
       "license": "MIT-0"
     },
     "node_modules/@csstools/css-tokenizer": {
-      "version": "3.0.4",
-      "resolved": "https://registry.npmjs.org/@csstools/css-tokenizer/-/css-tokenizer-3.0.4.tgz",
-      "integrity": "sha512-Vd/9EVDiu6PPJt9yAh6roZP6El1xHrdvIVGjyBsHR0RYwNHgL7FJPyIIW4fANJNG6FtyZfvlRPpFI4ZM/lubvw==",
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/css-tokenizer/-/css-tokenizer-4.0.0.tgz",
+      "integrity": "sha512-QxULHAm7cNu72w97JUNCBFODFaXpbDg+dP8b/oWFAZ2MTRppA3U00Y2L1HqaS4J6yBqxwa/Y3nMBaxVKbB/NsA==",
       "dev": true,
       "funding": [
         {
@@ -1723,13 +1723,13 @@
       ],
       "license": "MIT",
       "engines": {
-        "node": ">=18"
+        "node": ">=20.19.0"
       }
     },
     "node_modules/@csstools/postcss-light-dark-function": {
-      "version": "2.0.11",
-      "resolved": "https://registry.npmjs.org/@csstools/postcss-light-dark-function/-/postcss-light-dark-function-2.0.11.tgz",
-      "integrity": "sha512-fNJcKXJdPM3Lyrbmgw2OBbaioU7yuKZtiXClf4sGdQttitijYlZMD5K7HrC/eF83VRWRrYq6OZ0Lx92leV2LFA==",
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/postcss-light-dark-function/-/postcss-light-dark-function-3.0.0.tgz",
+      "integrity": "sha512-s++V5/hYazeRUCYIn2lsBVzUsxdeC46gtwpgW6lu5U/GlPOS5UTDT14kkEyPgXmFbCvaWLREqV7YTMJq1K3G6w==",
       "dev": true,
       "funding": [
         {
@@ -1743,22 +1743,22 @@
       ],
       "license": "MIT-0",
       "dependencies": {
-        "@csstools/css-parser-algorithms": "^3.0.5",
-        "@csstools/css-tokenizer": "^3.0.4",
-        "@csstools/postcss-progressive-custom-properties": "^4.2.1",
-        "@csstools/utilities": "^2.0.0"
+        "@csstools/css-parser-algorithms": "^4.0.0",
+        "@csstools/css-tokenizer": "^4.0.0",
+        "@csstools/postcss-progressive-custom-properties": "^5.0.0",
+        "@csstools/utilities": "^3.0.0"
       },
       "engines": {
-        "node": ">=18"
+        "node": ">=20.19.0"
       },
       "peerDependencies": {
         "postcss": "^8.4"
       }
     },
     "node_modules/@csstools/postcss-progressive-custom-properties": {
-      "version": "4.2.1",
-      "resolved": "https://registry.npmjs.org/@csstools/postcss-progressive-custom-properties/-/postcss-progressive-custom-properties-4.2.1.tgz",
-      "integrity": "sha512-uPiiXf7IEKtUQXsxu6uWtOlRMXd2QWWy5fhxHDnPdXKCQckPP3E34ZgDoZ62r2iT+UOgWsSbM4NvHE5m3mAEdw==",
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/postcss-progressive-custom-properties/-/postcss-progressive-custom-properties-5.0.0.tgz",
+      "integrity": "sha512-NsJoZ89rxmDrUsITf8QIk5w+lQZQ8Xw5K6cLFG+cfiffsLYHb3zcbOOrHLetGl1WIhjWWQ4Cr8MMrg46Q+oACg==",
       "dev": true,
       "funding": [
         {
@@ -1775,7 +1775,7 @@
         "postcss-value-parser": "^4.2.0"
       },
       "engines": {
-        "node": ">=18"
+        "node": ">=20.19.0"
       },
       "peerDependencies": {
         "postcss": "^8.4"
@@ -1828,9 +1828,9 @@
       }
     },
     "node_modules/@csstools/utilities": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/@csstools/utilities/-/utilities-2.0.0.tgz",
-      "integrity": "sha512-5VdOr0Z71u+Yp3ozOx8T11N703wIFGVRgOWbOZMKgglPJsWA54MRIoMNVMa7shUToIhx5J8vX4sOZgD2XiihiQ==",
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/utilities/-/utilities-3.0.0.tgz",
+      "integrity": "sha512-etDqA/4jYvOGBM6yfKCOsEXfH96BKztZdgGmGqKi2xHnDe0ILIBraRspwgYatJH9JsCZ5HCGoCst8w18EKOAdg==",
       "dev": true,
       "funding": [
         {
@@ -1844,7 +1844,7 @@
       ],
       "license": "MIT-0",
       "engines": {
-        "node": ">=18"
+        "node": ">=20.19.0"
       },
       "peerDependencies": {
         "postcss": "^8.4"
@@ -12363,49 +12363,6 @@
         "@csstools/css-tokenizer": "^4.0.0"
       }
     },
-    "node_modules/stylelint/node_modules/@csstools/css-parser-algorithms": {
-      "version": "4.0.0",
-      "resolved": "https://registry.npmjs.org/@csstools/css-parser-algorithms/-/css-parser-algorithms-4.0.0.tgz",
-      "integrity": "sha512-+B87qS7fIG3L5h3qwJ/IFbjoVoOe/bpOdh9hAjXbvx0o8ImEmUsGXN0inFOnk2ChCFgqkkGFQ+TpM5rbhkKe4w==",
-      "dev": true,
-      "funding": [
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/csstools"
-        },
-        {
-          "type": "opencollective",
-          "url": "https://opencollective.com/csstools"
-        }
-      ],
-      "license": "MIT",
-      "engines": {
-        "node": ">=20.19.0"
-      },
-      "peerDependencies": {
-        "@csstools/css-tokenizer": "^4.0.0"
-      }
-    },
-    "node_modules/stylelint/node_modules/@csstools/css-tokenizer": {
-      "version": "4.0.0",
-      "resolved": "https://registry.npmjs.org/@csstools/css-tokenizer/-/css-tokenizer-4.0.0.tgz",
-      "integrity": "sha512-QxULHAm7cNu72w97JUNCBFODFaXpbDg+dP8b/oWFAZ2MTRppA3U00Y2L1HqaS4J6yBqxwa/Y3nMBaxVKbB/NsA==",
-      "dev": true,
-      "funding": [
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/csstools"
-        },
-        {
-          "type": "opencollective",
-          "url": "https://opencollective.com/csstools"
-        }
-      ],
-      "license": "MIT",
-      "engines": {
-        "node": ">=20.19.0"
-      }
-    },
     "node_modules/stylelint/node_modules/@csstools/media-query-list-parser": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/@csstools/media-query-list-parser/-/media-query-list-parser-5.0.0.tgz",
diff --git a/package.json b/package.json
index 0337fbadf..84431bb47 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,7 @@
     "@babel/core": "^7.29.0",
     "@babel/preset-env": "^7.29.0",
     "@babel/runtime": "^7.28.6",
-    "@csstools/postcss-light-dark-function": "^2.0.11",
+    "@csstools/postcss-light-dark-function": "^3.0.0",
     "@fluent/bundle": "^0.19.1",
     "@fluent/dom": "^0.10.2",
     "@metalsmith/layouts": "^3.0.0",