diff --git a/README.md b/README.md
index 849c49af..f22b7419 100644
--- a/README.md
+++ b/README.md
@@ -155,6 +155,7 @@ npm install themer
 1. An array of [`ColorSet` objects](#create-custom-colorsets), or string identifiers of [`themer`'s built-in color sets](#themer-color-sets)
 2. An array of [`Template` objects](#create-custom-templates), or string identifiers of [`themer`'s built-in templates](#themer-templates)
 3. A `RenderOptions` object used to specify the resolution of the outputted wallpaper images
+4. (Optional) an `OutputFileTransform` async generator function that transforms the files generated by the provided templates. This function runs between each template's `render` and `renderInstructions` functions.
 
 The objects yielded by the generator are `OutputFile`s.
 
diff --git a/cli/src/index.ts b/cli/src/index.ts
index 65c66362..99981b51 100644
--- a/cli/src/index.ts
+++ b/cli/src/index.ts
@@ -21,4 +21,5 @@ export {
   allBuiltInTemplates,
   type BuiltInTemplate,
 } from './template/all.js';
+export { type OutputFileTransform } from './transform/index.js';
 export { themer as default } from './themer.js';
diff --git a/cli/src/themer.spec.ts b/cli/src/themer.spec.ts
new file mode 100644
index 00000000..b13594c1
--- /dev/null
+++ b/cli/src/themer.spec.ts
@@ -0,0 +1,46 @@
+import test from 'ava';
+import template from './fixture/template.js';
+import { themer } from './themer.js';
+import { OutputFileTransform } from './transform/index.js';
+import { basename } from 'node:path';
+
+const backupFile: OutputFileTransform = async function* (file) {
+  yield file;
+  yield {
+    ...file,
+    path: `${file.path}.bak`,
+  };
+};
+
+test('supports file post-processing', async (t) => {
+  const files = [];
+  for await (const file of themer(
+    ['default'],
+    [template],
+    { wallpaperSizes: [] },
+    backupFile,
+  )) {
+    files.push(file);
+  }
+
+  t.plan(9);
+
+  const expectedPaths = [
+    'Default/Test/themer-default-dark.txt',
+    'Default/Test/themer-default-dark.txt.bak',
+    'Default/Test/themer-default-light.txt',
+    'Default/Test/themer-default-light.txt.bak',
+  ];
+
+  const readme = files.find((file) => file.path.endsWith('README.md'));
+  t.assert(readme, 'No README file in output');
+
+  for (const path of expectedPaths) {
+    t.assert(
+      files.find((file) => file.path === path),
+      `Output files must contain ${path}`,
+    );
+    const base = basename(path);
+    t.assert(readme?.content.includes(base), `README must contain ${base}`);
+  }
+});
diff --git a/cli/src/themer.ts b/cli/src/themer.ts
index 78a82eda..665e7582 100644
--- a/cli/src/themer.ts
+++ b/cli/src/themer.ts
@@ -2,11 +2,13 @@ import { ColorSet, prepareColorSet } from './color-set/index.js';
 import { BuiltInColorSet, resolveColorSet } from './color-set/all.js';
 import type { Template, OutputFile, RenderOptions } from './template/index.js';
 import { BuiltInTemplate, resolveTemplate } from './template/all.js';
+import { OutputFileTransform, noopTransform } from './transform/index.js';
 
 export async function* themer(
   colorSets: (BuiltInColorSet | ColorSet)[],
   templates: (BuiltInTemplate | Template)[],
   options: RenderOptions,
+  transform: OutputFileTransform = noopTransform,
 ): AsyncGenerator<OutputFile> {
   for (const colorSet of colorSets) {
     const resolvedColorSet = resolveColorSet(colorSet);
@@ -16,13 +18,15 @@ export async function* themer(
     const rootDir = fullColorSet.name;
     for (const template of resolvedTemplates) {
       const templatePaths: string[] = [];
-      for await (const file of template.render(fullColorSet, options)) {
-        const path = `${template.name}/${file.path}`;
-        yield {
-          ...file,
-          path: `${rootDir}/${path}`,
-        };
-        templatePaths.push(path);
+      for await (const renderedFile of template.render(fullColorSet, options)) {
+        for await (const file of transform(renderedFile)) {
+          const path = `${template.name}/${file.path}`;
+          yield {
+            ...file,
+            path: `${rootDir}/${path}`,
+          };
+          templatePaths.push(path);
+        }
       }
       instructions.push(`## ${template.name}`);
       instructions.push(
diff --git a/cli/src/transform/index.ts b/cli/src/transform/index.ts
new file mode 100644
index 00000000..210e849c
--- /dev/null
+++ b/cli/src/transform/index.ts
@@ -0,0 +1,9 @@
+import { OutputFile } from '../template/index.js';
+
+export type OutputFileTransform = (
+  file: OutputFile,
+) => AsyncGenerator<OutputFile>;
+
+export const noopTransform: OutputFileTransform = async function* (file) {
+  yield file;
+};