It depends on the context and how the art style is being copied. If someone is copying an art style without permission or proper attribution, then it can be considered stealing or plagiarism. This is especially true if the original artist's work is copied without any transformative changes or new creative elements being added. However, if the copying is done with permission or the artist uses an art style as inspiration to create something new and original, it may not be considered stealing. In fact, many artists draw inspiration from other artists and art styles to create their own unique work. It's important to always give credit where credit is due and to be respectful of other artists' work.