You might be using plain text mode if you are unable to add text formatting to your signature. To stop using plain text:

To start a new message, choose Compose.

Compose is highlighted in your Gmail inbox. Click More choices (three dots).

gmail compose window with the highlighted More option (three dots) Take the checkmark off of Plain text mode.

Plain Text Mode is highlighted in the Gmail compose box. The Signature for Multiple Accounts Can Be Changed Give each email address a unique signature if you use several Gmail accounts or the Send mail as function. Go to the Signature area and choose the desired signature from the drop-down list to alter the signature for a separate account.