Default fonts of your email signature can be changed in the Preferences pane in Outlook toolbar. To change the default fonts, follow the steps below:

1. Go to Preferences in the Outlook  toolbar.
   
2. Select the Signatures options. 
   
3. Click inside the Signature box to edit your email signature. Highlight the area you wish to change and then Right Click and select Font. 
   
4. Select Helvetica Font, with an Automatic Colour and Size 11 font. Press OK to save these changes.
   
5. Once you are finished editing your signature, it will save automatically. 

To change the default font for new emails, replies and forwards, follow the steps in the FAQ here.