Here is how to add an HTML signature in Apple Mail:
Under Mail >> Preferences
Create an empty Signature
Turn off / Close Apple Mail
In the Finder Navigation (top of screen), click on Go, while pressing "Option"; Library will appear. Clicking on the Library list item.
In the Library folder, click on Mail >>V7 (or V8 depending on your OS version) >> MailData >> Signatures >> file ending with .mailsignature
Open the file with TextEdit
Remove unnecessary code, adding your HTML signature between the opening and closing <body> tags.
Save + Lock the file.
Re-open Apple Mail and test your signature.