Not that answer that you're looking for - there is no Tableau desktop config file that you can change a master font.
Create a new workbook, set your workbook defaults as you prefer, create a sheet and dashboard (just blank, no data) and save as your template.
There's a great article on custom color palettes.
Hope I could help.
I came here to ask a similar question. Every time I create a new dashboard, I have to recreate the header, footer, add company logo, etc. Is there any way to have these always be there when a new dashboard sheet is created? I expect no...
It's surprising that an enterprise software like this doesn't support company formatting standards, especially having a decent server component.