OK, you've created a new style, and you want to make it available as the default. How do you do this?
Easy! You may have noticed some checkboxes at the top left of the Style Manager window.
The style currently in use is checked. To change the style in use, just swap the check marks around, and click on the button marked Save Display Order.
The new style will now be available, and the old style won't be available. If you want to make both styles available, check both options and click on Save Display Order.
This will ...