How Do You Horizontally Center the Contents in a Google Apps Script Flextable?

I am building my very first Google Scripts Web App.

I am using a FlexTable in a Google Apps Script to display my content.

I have been merrily using the following to set a variety of CSS styles for my content in the cells:

FlexTable.setStyleAttributes(row,col,{attribute: "value"});

However, it does not seem to respond to marginLeft nor marginRight at all. How can I center the contents of a FlexTable cell?