diff --git a/mobile/scripts/build-terminal-document-fixture.mjs b/mobile/scripts/build-terminal-document-fixture.mjs
index f4c65a09c49..885aca358d7 100644
--- a/mobile/scripts/build-terminal-document-fixture.mjs
+++ b/mobile/scripts/build-terminal-document-fixture.mjs
@@ -1,6 +1,6 @@
 import { writeFile } from 'node:fs/promises'
 import path from 'node:path'
-import * as esbuild from 'esbuild'
+import { importTypeScriptModule } from './import-typescript-module.mjs'
 
 /**
  * Writes the committed copy of the terminal WebView document that
@@ -36,19 +36,6 @@ export const TERMINAL_DOCUMENT_FIXTURE_PATH = path.join(
 export const ENGINE_JS_PLACEHOLDER = '__ORCA_TERMINAL_ENGINE_JS__'
 export const ENGINE_CSS_PLACEHOLDER = '__ORCA_TERMINAL_ENGINE_CSS__'
 
-async function loadModule(entryPoint) {
-  const result = await esbuild.build({
-    entryPoints: [entryPoint],
-    bundle: true,
-    format: 'esm',
-    platform: 'node',
-    write: false,
-    logLevel: 'silent'
-  })
-  const code = result.outputFiles[0].text
-  return import(`data:text/javascript;base64,${Buffer.from(code, 'utf8').toString('base64')}`)
-}
-
 /**
  * The document with both generated sections replaced by their placeholders.
  *
@@ -76,8 +63,8 @@ export function terminalDocumentFixture(document, engineJs, engineCss) {
 
 async function main() {
   const [{ XTERM_HTML }, { XTERM_ENGINE_JS, XTERM_ENGINE_CSS }] = await Promise.all([
-    loadModule(entry),
-    loadModule(enginePath)
+    importTypeScriptModule(entry),
+    importTypeScriptModule(enginePath)
   ])
   const fixture = terminalDocumentFixture(XTERM_HTML, XTERM_ENGINE_JS, XTERM_ENGINE_CSS)
   await writeFile(TERMINAL_DOCUMENT_FIXTURE_PATH, fixture)
diff --git a/mobile/scripts/build-terminal-document-script.mjs b/mobile/scripts/build-terminal-document-script.mjs
index f4b045541ef..ccdb8721d60 100644
--- a/mobile/scripts/build-terminal-document-script.mjs
+++ b/mobile/scripts/build-terminal-document-script.mjs
@@ -1,5 +1,7 @@
 import { readFile } from 'node:fs/promises'
+import path from 'node:path'
 import * as esbuild from 'esbuild'
+import { importTypeScriptModule } from './import-typescript-module.mjs'
 
 /**
  * Turns one module of the in-WebView terminal document back into the script text the document
@@ -20,6 +22,34 @@ import * as esbuild from 'esbuild'
  */
 const INDENT = '  '
 
+const constantsPath = path.join(
+  import.meta.dirname,
+  '..',
+  'src',
+  'terminal',
+  'document',
+  'document-constants.ts'
+)
+
+let substitutions = null
+
+/**
+ * `document-constants.ts` as esbuild `define` entries.
+ *
+ * Substitution happens after the import lines are dropped, when the names are free again; while the
+ * import is still there esbuild sees a bound name and leaves it alone, which is the correct thing
+ * for the page and the wrong thing for the document.
+ */
+async function documentConstantSubstitutions() {
+  if (substitutions === null) {
+    const module = await importTypeScriptModule(constantsPath)
+    substitutions = Object.fromEntries(
+      Object.entries(module).map(([name, value]) => [name, JSON.stringify(value)])
+    )
+  }
+  return substitutions
+}
+
 /** Whether a line opens an import the document does not need. */
 function isImportLine(line) {
   return /^import[\s{'"]/.test(line)
@@ -68,7 +98,15 @@ export async function emitTerminalDocumentModule(modulePath) {
     }
     kept.push(line.startsWith('export ') ? line.slice('export '.length) : line)
   }
-  const body = kept.join('\n').trim()
+  const define = await documentConstantSubstitutions()
+  const substituted = await esbuild.transform(kept.join('\n'), {
+    loader: 'js',
+    format: 'esm',
+    target: 'chrome74',
+    minify: false,
+    define
+  })
+  const body = substituted.code.trim()
   return body
     .split('\n')
     .map((line) => (line.length === 0 ? line : `${INDENT}${line}`))
diff --git a/mobile/scripts/build-terminal-document-script.test.ts b/mobile/scripts/build-terminal-document-script.test.ts
index 9d223e2cdf8..0520176a738 100644
--- a/mobile/scripts/build-terminal-document-script.test.ts
+++ b/mobile/scripts/build-terminal-document-script.test.ts
@@ -3,6 +3,7 @@ import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { afterAll, beforeAll, describe, expect, it } from 'vitest'
 import { emitTerminalDocumentModule } from './build-terminal-document-script.mjs'
+import { terminalBackgroundFallback } from '../src/terminal/document/document-constants'
 
 /**
  * What the generator drops, what it keeps, and how it puts a module back into the document.
@@ -61,4 +62,15 @@ describe('emitting one terminal document module', () => {
       )
     ).toBe('  function f(v) {\n    return v.a;\n  }')
   })
+
+  it('substitutes a build-time constant the document carries as a literal', async () => {
+    const emitted = await emit(
+      "import { terminalBackgroundFallback } from '../src/terminal/document/document-constants'\n" +
+        'export function paint() {\n' +
+        '  return terminalBackgroundFallback\n' +
+        '}\n'
+    )
+    expect(emitted).toContain(JSON.stringify(terminalBackgroundFallback))
+    expect(emitted).not.toContain('terminalBackgroundFallback')
+  })
 })
diff --git a/mobile/scripts/import-typescript-module.mjs b/mobile/scripts/import-typescript-module.mjs
new file mode 100644
index 00000000000..54f84ab800b
--- /dev/null
+++ b/mobile/scripts/import-typescript-module.mjs
@@ -0,0 +1,21 @@
+import * as esbuild from 'esbuild'
+
+/**
+ * Imports a TypeScript module from a build script, by bundling it to a data URL.
+ *
+ * Node cannot import TypeScript and these scripts run outside the app's bundler, so the values the
+ * document is built from — the theme, the URL limits, the caret options — would otherwise have to be
+ * restated here. Restating them is what the generator exists to avoid.
+ */
+export async function importTypeScriptModule(entryPoint) {
+  const result = await esbuild.build({
+    entryPoints: [entryPoint],
+    bundle: true,
+    format: 'esm',
+    platform: 'node',
+    write: false,
+    logLevel: 'silent'
+  })
+  const code = result.outputFiles[0].text
+  return import(`data:text/javascript;base64,${Buffer.from(code, 'utf8').toString('base64')}`)
+}
diff --git a/mobile/src/terminal/document/document-constants.ts b/mobile/src/terminal/document/document-constants.ts
new file mode 100644
index 00000000000..003ab55b741
--- /dev/null
+++ b/mobile/src/terminal/document/document-constants.ts
@@ -0,0 +1,15 @@
+import { colors } from '../../theme/mobile-theme'
+
+/**
+ * The build-time values the document's script text carries as literals.
+ *
+ * The document is a string, so it cannot import: today each of these is interpolated into a
+ * template literal at the site that needs it. A module cannot do that and still be the same
+ * program, so the generator substitutes these exports into the text it emits, and the web page
+ * imports the very same bindings. One source either way.
+ *
+ * Every export must be JSON-serialisable, because a substitution is a JSON literal.
+ */
+
+/** The page background before a theme arrives, and the fallback when a theme omits one. */
+export const terminalBackgroundFallback = colors.terminalBg