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.