Hi,

Go to Settings > General Settings > Document Layout as shown in the screenshot.

Choose a layout style like Boxed or Bold.

Select the red color from the color palette on the left side.

Click the Save button at the bottom of the layout popup (make sure it's confirmed).

Click the Download PDF Preview link to verify if red boxes appear in the example.

If the preview works but the real invoice PDF doesn’t, go to an actual invoice and print it using Print > Invoice.

If styles are still missing, confirm your server has wkhtmltopdf 0.12.5 (patched) installed using: wkhtmltopdf --version.

Clear your browser cache or open in incognito mode to make sure cached PDFs aren’t misleading you.

If you have any third-party module installed, disable it temporarily and test the default Odoo layout.

For advanced invoice layout editing, consider using this module: https://apps.odoo.com/apps/modules/17.0/invoice_format_editor

Hope it helps.