DocumentationMathematicaFront EndStyle SheetsDefault Style Sheets
Setting the Default Style Sheet Option
The assignment for the default style sheet is set as an option in the file FrontEnd/init.m in your preferences directory. The option is DefaultStyleDefinitions :> "filename", where filename is a shared style sheet or a style sheet that you have created. In some cases, the front end stores the file name using the FrontEnd`FileName command.