You can find Outlook customization options in settings ... You can also change the font, size, color and format of your signature using the options menu below the signature box.

The process to add a signature in Outlook.com is different from the process to add a signature in the Outlook desktop app. The following steps will guide you on how to add a signature in Outlook.com.

and then add it to your email's signature creation tool. Microsoft's support site offers a tutorial on how to create signatures in Microsoft Outlook.