To change the format of HTML body you can try writing like this.
string Text = "<html><div style=""font-size:10.5px; font-family:Tahoma;"">" + mailItem.HTMLBody + "</div></html>"
The other approach is to replace the default font-family with your desired font-family.
In this you can do like this.
mailItem.HTMLBody = mailItem.HTMLBody.Replace("Times New Roman", "Tahoma");
But I am not sure how far it is effective to write like this.