Tableau allows you to set certain formatting defaults at a workbook level. To view the options and make changes to the defaults, click Format | Workbook.... The left pane will now show formatting options for the workbook:

The options include the ability to change default Fonts, which they apply to various parts of a view or dashboard, and default Lines, which apply to the various types of lines used in visualizations. Notice also the Reset to Defaults button, should you wish to revert to the default formatting.