diff --git a/superset-frontend/src/features/themes/ThemeModal.test.tsx b/superset-frontend/src/features/themes/ThemeModal.test.tsx
index 4994863b074..2b4c29efc4f 100644
--- a/superset-frontend/src/features/themes/ThemeModal.test.tsx
+++ b/superset-frontend/src/features/themes/ThemeModal.test.tsx
@@ -712,6 +712,138 @@ test('applies theme locally when clicking Apply button', async () => {
   expect(mockThemeContext.setTemporaryTheme).toHaveBeenCalled();
 });
 
+test('shows Format button when modal is in edit mode', () => {
+  render(
+    <ThemeModal
+      addDangerToast={jest.fn()}
+      addSuccessToast={jest.fn()}
+      onThemeAdd={jest.fn()}
+      onHide={jest.fn()}
+      show
+      canDevelop={false}
+    />,
+    { useRedux: true, useRouter: true },
+  );
+
+  expect(screen.getByRole('button', { name: /format/i })).toBeInTheDocument();
+});
+
+test('does not show Format button for read-only system themes', async () => {
+  render(
+    <ThemeModal
+      addDangerToast={jest.fn()}
+      addSuccessToast={jest.fn()}
+      onThemeAdd={jest.fn()}
+      onHide={jest.fn()}
+      show
+      canDevelop={false}
+      theme={mockSystemTheme}
+    />,
+    { useRedux: true, useRouter: true },
+  );
+
+  await screen.findByText('System Theme - Read Only');
+
+  expect(
+    screen.queryByRole('button', { name: /format/i }),
+  ).not.toBeInTheDocument();
+});
+
+test('disables Format button when JSON is invalid', async () => {
+  render(
+    <ThemeModal
+      addDangerToast={jest.fn()}
+      addSuccessToast={jest.fn()}
+      onThemeAdd={jest.fn()}
+      onHide={jest.fn()}
+      show
+      canDevelop={false}
+    />,
+    { useRedux: true, useRouter: true },
+  );
+
+  const jsonEditor = screen.getByTestId('json-editor');
+  userEvent.clear(jsonEditor);
+  userEvent.type(jsonEditor, '{invalid json');
+
+  await waitFor(() => {
+    expect(screen.getByRole('button', { name: /format/i })).toBeDisabled();
+  });
+});
+
+test('enables Format button when JSON is valid', async () => {
+  render(
+    <ThemeModal
+      addDangerToast={jest.fn()}
+      addSuccessToast={jest.fn()}
+      onThemeAdd={jest.fn()}
+      onHide={jest.fn()}
+      show
+      canDevelop={false}
+    />,
+    { useRedux: true, useRouter: true },
+  );
+
+  await addValidJsonData();
+
+  await waitFor(() => {
+    expect(screen.getByRole('button', { name: /format/i })).toBeEnabled();
+  });
+});
+
+test('Format button pretty-prints minified JSON', async () => {
+  render(
+    <ThemeModal
+      addDangerToast={jest.fn()}
+      addSuccessToast={jest.fn()}
+      onThemeAdd={jest.fn()}
+      onHide={jest.fn()}
+      show
+      canDevelop={false}
+    />,
+    { useRedux: true, useRouter: true },
+  );
+
+  const minifiedJson = '{"token":{"colorPrimary":"#1890ff"}}';
+  const jsonEditor = screen.getByTestId('json-editor');
+  userEvent.clear(jsonEditor);
+  userEvent.type(jsonEditor, minifiedJson);
+
+  const formatButton = screen.getByRole('button', { name: /format/i });
+  userEvent.click(formatButton);
+
+  const expectedFormatted = JSON.stringify(
+    { token: { colorPrimary: '#1890ff' } },
+    null,
+    2,
+  );
+  await waitFor(() => {
+    expect(jsonEditor).toHaveValue(expectedFormatted);
+  });
+});
+
+test('Format button is disabled when JSON editor is empty', async () => {
+  render(
+    <ThemeModal
+      addDangerToast={jest.fn()}
+      addSuccessToast={jest.fn()}
+      onThemeAdd={jest.fn()}
+      onHide={jest.fn()}
+      show
+      canDevelop={false}
+    />,
+    { useRedux: true, useRouter: true },
+  );
+
+  // The editor initializes with `{}` — clear it to reach the empty state
+  const jsonEditor = screen.getByTestId('json-editor');
+  userEvent.clear(jsonEditor);
+
+  await waitFor(() => {
+    expect(screen.getByRole('button', { name: /format/i })).toBeDisabled();
+  });
+});
+
 test('disables Apply button when JSON configuration is invalid', async () => {
   fetchMock.clearHistory().removeRoutes();
   fetchMock.get('glob:*/api/v1/theme/*', {
diff --git a/superset-frontend/src/features/themes/ThemeModal.tsx b/superset-frontend/src/features/themes/ThemeModal.tsx
index 59d843d1d90..db7692443bb 100644
--- a/superset-frontend/src/features/themes/ThemeModal.tsx
+++ b/superset-frontend/src/features/themes/ThemeModal.tsx
@@ -71,6 +71,15 @@ const toEditorAnnotations = (
     message: ann.text,
   }));
 
+const formatJsonData = (jsonData?: string): string | undefined => {
+  if (!jsonData) return jsonData;
+  try {
+    return JSON.stringify(JSON.parse(jsonData), null, 2);
+  } catch {
+    return jsonData;
+  }
+};
+
 interface ThemeModalProps {
   addDangerToast: (msg: string) => void;
   addSuccessToast?: (msg: string) => void;
@@ -316,6 +325,15 @@ const ThemeModal: FunctionComponent<ThemeModalProps> = ({
     [currentTheme],
   );
 
+  const onFormat = useCallback(() => {
+    if (currentTheme?.json_data) {
+      const formatted = formatJsonData(currentTheme.json_data);
+      if (formatted !== currentTheme.json_data) {
+        onJsonDataChange(formatted || '');
+      }
+    }
+  }, [currentTheme?.json_data, onJsonDataChange]);
+
   const validate = () => {
     if (isReadOnly || !currentTheme) {
       setDisableSave(true);
@@ -357,8 +375,12 @@ const ThemeModal: FunctionComponent<ThemeModalProps> = ({
 
   useEffect(() => {
     if (resource) {
-      setCurrentTheme(resource);
-      setInitialTheme(resource);
+      const formatted = {
+        ...resource,
+        json_data: formatJsonData(resource.json_data),
+      };
+      setCurrentTheme(formatted);
+      setInitialTheme(formatted);
     }
   }, [resource]);
 
@@ -522,27 +544,47 @@ const ThemeModal: FunctionComponent<ThemeModalProps> = ({
                 annotations={toEditorAnnotations(validation.annotations)}
               />
             </StyledEditorWrapper>
-            {canDevelopThemes && (
-              <div className="apply-button-container">
-                <Tooltip
-                  title={t('Set local theme for testing (preview only)')}
-                  placement="top"
-                >
-                  <Button
-                    icon={<Icons.ThunderboltOutlined />}
-                    onClick={onApply}
-                    disabled={
-                      !currentTheme?.json_data ||
-                      !isValidJson(currentTheme.json_data) ||
-                      validation.hasErrors
-                    }
-                    buttonStyle="secondary"
+            <div className="apply-button-container">
+              <Space>
+                {!isReadOnly && (
+                  <Tooltip
+                    title={t('Format JSON configuration')}
+                    placement="top"
                   >
-                    {t('Apply')}
-                  </Button>
-                </Tooltip>
-              </div>
-            )}
+                    <Button
+                      icon={<Icons.AlignLeftOutlined />}
+                      buttonStyle="secondary"
+                      onClick={onFormat}
+                      disabled={
+                        !currentTheme?.json_data ||
+                        !isValidJson(currentTheme.json_data)
+                      }
+                    >
+                      {t('Format')}
+                    </Button>
+                  </Tooltip>
+                )}
+                {canDevelopThemes && (
+                  <Tooltip
+                    title={t('Set local theme for testing (preview only)')}
+                    placement="top"
+                  >
+                    <Button
+                      icon={<Icons.ThunderboltOutlined />}
+                      onClick={onApply}
+                      disabled={
+                        !currentTheme?.json_data ||
+                        !isValidJson(currentTheme.json_data) ||
+                        validation.hasErrors
+                      }
+                      buttonStyle="secondary"
+                    >
+                      {t('Apply')}
+                    </Button>
+                  </Tooltip>
+                )}
+              </Space>
+            </div>
           </Form.Item>
         </Form>
       </StyledFormWrapper>