Hi @Abel Tana ,
Welcome to our forum!
I want to change Calibri to Arial but it seems that i've already set as default all fonts in outlook to Arial but the changes are not applied to the signatures (when i create a new one the default template is in Calibri).
I indeed tested and researched a lot to try changing the default Signature font in Outlook but failed to find an effective method to change this. Just got the same test results as yours, Outlook default email message font and the default Signature font are two different settings. Tried using custom stationary and themes, manually changing the default email message font, but both of them will not influence the default Signature font. Tried Process Monitor to observe which registry values will change when modifying the signature font, but also could not get much helpful information. I also did a lot of search about Outlook signature-related Registry value, however, negative as well.
So, I'm afraid that we are not able to change this default Signature font by ourselves.
Here is some other Signature Registry Settings I found, for your reference:
Outlook Signature Registry Settings
How to: Disable End-User's Ability to Add, Edit, & Remove Outlook Signatures
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