Mac: the file
Download the .mailsignature file from Outromail. Quit Mail. Put the file in ~/Library/Mail/V10/MailData/Signatures (the V number varies by macOS). Open Mail; the signature appears under Settings, Signatures.
Mac: paste
Alternatively, Mail, Settings, Signatures, +, then paste the copied signature. Untick “Always match my default message font”.
iPhone and iPad
Copy the signature from Outromail in Safari, then Settings, Mail, Signature, paste. iOS keeps the formatting.
Dark mode
Apple Mail shows the card as designed in dark mode when the background is set explicitly, which Outromail does.