Yes, it is possible to set default style for any element type. You can do it this way:
1. In ribbon, open Design tab, click on bottom part of Style Sets button to show its drop down menu.
2. In menu, click on Edit Current Style Set.
Style Set Editor will show.
3. In this window, click on Green Plus button to add a new style.
4. New Style form will show - Choose 'For specific element' option.
5. In second drop down for 'Element type', select an element type (Flow) for which you want to set a default style.
6. Confirm the dialog clicking on OK button.
7. A new style will be created and it is open for editing on the right.
8. Change the style as you want in Style tab (right side of the window).
9. Click on OK button.
I hope I helped you.