There are various Content Snippets you can add or update (in the Portal/Power Pages Management App check if the content snippet exists, if so edit it, if not create it)
Account/Register/PageCopy
Account/Register/RegisterLocalFormHeading
Account/Register/EmailLabel
Account/Register/UsernameLabel
Account/Register/PasswordLabel
Account/Register/ConfirmPasswordLabel
Account/Register/RegisterExternalFormHeading
Account/Register/RegisterButtonText
Account/Register/RegistrationDisabledMessage
Most of them will be text (page copy can be HTML)
For additional customization you are very limited with the sign-in and registration pages, basically find a content snippet that will take HTML and make the update using JavaScript/JQuery from inside that snippet.
After adding/updating a Content Snippet clear the portal cache (in the Designer Sync and then preview, manually <yoursiteurl>/_services/about when logged in as a user with the Administrators web role).