feature request: Add support for changing the default font and adding themes

[jmcnamara]

I plan to implement a feature that will allow users to change the default font in a workbook and also to change the default theme.

I had avoided this in the past because changing the default font changes the dimensions of rows and columns and that has a knock-on effect on the sizing of worksheet objects such as images and charts.

I recently figured out how to change the default font and maintain image scaling by using simple metrics that can be supplied by the user.

See also #994, #606, #472, #140