Is it concidered "art theft" if I try and use someone else's art style?

Not art theft, but maybe copyright concerns, depending on if the artist copyrighted the art before you use it. I believe art theft is actually the art of stealing priceless art, like the Mona Lisa and items like it. Of course, I could be wrong.

No, it's not a plagiarizing or copyright issue if you mimic a style for your own art. However, doing so is generally deemed to show lack of creativity, and some even think it's lazy. If you're doing it for your own purposes then it's fine, it doesn't matter what you draw or how you draw it. But if you're trying to sell or make a name for yourself it won't be received as well.

no not at all infact lots of famous art works and their styles have been replicated since well basically the begining of time

think about this manga is an art style look at all the manga being drawn. or the term graphitti style

if you want to make something new take all the styles you like and combine them to make a new one. its a secret.. ;)

No

No. It's inspiration. :)

No.