On the topic of thinning polishes using acetone, i have to advise against it. While acetone may work to thin the polish, it can also be damaging to the chemical composition of the polish, potentially harming the wear and shine of the product.
Using nail polish thinner (available at your local nail or beauty supply store, Ulta, Sally Beauty Supply, or via and etailer like head2toebeauty) is the best option. Thinner is specifically designed to replace the solvents that have evaporated from your polish, which is what causes it to thicken.
You'll notice that acetone is not on the original ingredients list of your polish, but ingredients like Butyl Acetate and Ethyl Acetate (common thinner ingredients) are two of the most common polish ingredients. That's why they're in your thinner, because as i mentioned, the purpose of thinner is to replace these solvents.
So, i know that its a common misconception that using Acetone or polish remover to thin your polish is ok, but i seriously advise you to invest in a bottle of polish thinner instead. Using thinner will not damage your polish, just remember to use it sparingly so you don't over-thin. Many thinners, such as my favorite, Seche Restore, come with a small glass eyedropper to help you control the amount you add. While Restore is my favorite, it does contain Toluene (one of the Big Three chemicals). If you'd prefer a thinner without Toluene, Beauty Secrets thinner is available at Sally Beauty Supply, and doesn't contain Toluene.
I hope this helps dispel the myth that its fine to use acetone as a polish thinning agent! I wouldn't want to see any of my lovely readers tossing a precious pretty because it became a dull, chippy mess every time they tried to wear it :)