LibreOffice Calc – How to Set Default Font?

If you want to use the customized document as standard template (so it's used if you select menu File -> New -> Spreadsheet), the following steps are required (assuming you've created a fresh spreadsheet document and changed the default font as Kirk has described):

• Select menu File -> Templates -> Save...;
• In the Template dialogue, enter a name for your template; make sure that "My Templates" is selected in the "Categories" list, then click OK.
• Select menu File -> Templates -> Organize...;
• In the template management dialogue, double-click on the "My Templates" entry in the list on the left; below "My Templates", a list of your templates should appear, among them the template you saved in step 2;
• right-click on our template defined in step 2, select "Set as default template", close the template management dialogue.

That's it - now, if you create a new spreadsheet document, it should be based on your custom template.

EDIT 1:

For updated instructions based on LibreOffice 4, see carnendil's answer

EDIT 2:

Setting a custom default template doesn't affect CSV files opened with LO Calc. There are two workarounds:

• use csv2odf to create a new ods file from csv and name the Calc template to use explicitly (see https://stackoverflow.com/a/13446079/342546);

• create a new ods file and "import" the csv data using Menu Insert -> Sheet from File or Menu Insert -> Sheet -> from file (see this answer on Ask LibreOffice):