Files with the .xtreme file extension most commonly contain desktop theme files that have been created by the Winstep Xtreme software application. Winstep Xtreme is a program that is used to create custom desktop environments for Windows users. The XTREME files contain the themes that can be used to alter the appearance of the Windows interface. These files are saved in a compressed file format, saving disk space and allowing for faster download times.
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If you have additional information about the XTREME file format or software that uses files with the XTREME suffix, please do get in touch - we would love hearing from you.