diff --git a/packages/tokens/scripts/build_tokens.mjs b/packages/tokens/scripts/build_tokens.mjs
index cdc73e3..c4e92a7 100644
--- a/packages/tokens/scripts/build_tokens.mjs
+++ b/packages/tokens/scripts/build_tokens.mjs
@@ -42,6 +42,7 @@ import {
   selectColorValuePreprocessor,
   convertClampStringToDimension,
 } from "./build_tokens_preprocessors.js";
+import { DARK_SELECTOR, LIGHT_SELECTOR } from "../src/color_mode.js";
 
 /**
  * Design tokens
@@ -234,7 +235,7 @@ const getStyleDictionaryConfigDefault = (buildPath) => (
             destination: "tokens.css",
             format: "css/variables",
             options: {
-              selector: ":root, .gl-light-scope",
+              selector: LIGHT_SELECTOR,
             },
           },
         ],
@@ -370,7 +371,7 @@ const getStyleDictionaryConfigDarkMode = (buildPath) => merge(getStyleDictionary
           {
             destination: "tokens.dark.css",
             options: {
-              selector: ":root.gl-dark, .gl-dark-scope",
+              selector: DARK_SELECTOR,
             },
           },
         ],
diff --git a/packages/tokens/src/color_mode.js b/packages/tokens/src/color_mode.js
new file mode 100644
index 0000000..20059f3
--- /dev/null
+++ b/packages/tokens/src/color_mode.js
@@ -0,0 +1,16 @@
+/**
+ * The classes that decide which set of token values applies to an element.
+ *
+ * Dark mode is applied either to the whole document, by putting `gl-dark` on the root, or to a
+ * subtree, by putting `gl-dark-scope` on any ancestor. `gl-light-scope` flips a subtree back.
+ *
+ * These are the single source of truth: `scripts/build_tokens.mjs` builds the CSS selectors from
+ * them, so the stylesheets and any JavaScript that has to reason about the colour mode cannot drift
+ * apart. Changing a name here changes both.
+ */
+export const DARK_ROOT_CLASS = "gl-dark";
+export const DARK_SCOPE_CLASS = "gl-dark-scope";
+export const LIGHT_SCOPE_CLASS = "gl-light-scope";
+
+export const LIGHT_SELECTOR = `:root, .${LIGHT_SCOPE_CLASS}`;
+export const DARK_SELECTOR = `:root.${DARK_ROOT_CLASS}, .${DARK_SCOPE_CLASS}`;