Default Font & Size in Google Sheets

[capper]

I have created a multi-tab Google Sheets document.  I would like to maintain a certain font and font size each and every time I edit the document or those who have access edit it.  For some reason I am having great difficulty as it seems to revert back to Arial 10 often.  How can I create a default that will stay put?  I am sure that by highlighting all existing data, I can switch it to the default of my choice.  it is the new entry material that doesn't seem to want to stick to a default I have chosen.  Any support would be great, thanks!

[Geek-9pm]

Google says:

Every cell in a new spreadsheet uses the same default formatting. As you begin to build a spreadsheet, you can customize the formatting to make your information easier to view and understand. In our example, we will be using a spreadsheet to plan and organize a garden plot.

Source:
https://edu.gcfglobal.org/en/googlespreadsheets/formatting-cells/1/

Check the video on that site.