How do I go about making sure my idea is safe before I design it and can I patent the idea?

You may be able to apply for a design patent instead of a utility patent. Have anyone who sees or learns of your idea sign a nondisclosure agreement to protect your idea, including anyone you hire to create the product, if you can't do it yourself, until you fill an application.

Generally, a new use for an existing product is patentable. Also, most inventions are combinations of existing products. A good start would be a thorough patent search to determine if anyone else has any patent rights or a patent application that might limit your invention. While you can do some searching yourself, it is recommended that you seek the advice of counsel to assist you. While there can never be absolute certainty, counsel can help to minimize your risk.

Market and style may not protect you from a charge of patent infringement if the patent on the original item is still in force. Check the Patent Office website for the status of the original patent. If the patent has expired - you are free to make, use and sell the item. As for obtaining your own patent - your version of the invention would need to pass these tests - especially as compared to the original patented item - (1) novelty (is your version "new" ?); (2) useful (does your item have a real-world use?); and (3) obviousness (is your version so different from the original that it is not a simple or "obvious" change?)

Ideas are never protectable thus they are never safe. Only the embodiment of the idea is protectable. If you have not made one yet - either constructive or actual embodiment - you have nothing to protect.