LibreOffice Calc is an excellent open-source replacement for the popular spreadsheet program Microsoft Excel. One irritation for me is that the default font and font size it uses are quite ugly.
To set a different font, font size and other text formatting as default I used these steps:
Open LibreOffice Calc. It opens with a new spreadsheet document.
Go to Styles -> Manage Styles. This opens Styles and Formatting sidebar. Right-click on Default and choose Modify.
In the Default dialog, set the formatting you would like. For example, you can set the font in the Font tab here.
We now save this format as a template: Go to File -> Templates -> Save As Template. Choose My Templates and click Save. Give your template a name.
Finally, we make our template as the default template: Go to File -> Templates -> Manage. In the Spreadsheets tab, click on your template and click Set as default.
All new spreadsheets created by Libreoffice Calc will now have the default font set in this template. Note that this is the font that will also be used for comments in Calc.
Tried with: LibreOffice 126.96.36.199 and Ubuntu 18.04