A check appears beside the stylesheet currently assigned to the notebook.
Mathematica standard-style notebooks are kept in SystemFiles/FrontEnd/StyleSheets. Style notebooks created by users can be put in Configuration/FrontEnd/StyleSheets. Any style notebooks in these directories will automatically be listed in the Stylesheet submenu.
Format ► Stylesheet ► Stylesheet Chooser command opens the Stylesheet palette which can either apply a stylesheet to an open notebook or open a new styled notebook.
Format ► Stylesheet ► Other command opens a dialog box to find stylesheets not listed in the Stylesheet submenu.
The commands in this submenu set the option StyleDefinitions on the selected notebook.