This tab displays the file content of the currently edited web part layout's theme folder and allows it to be managed. The theme folder of a layout is always located under the ~/App_Themes/Components/WebParts/<web part code name>/Layouts/ directory and its name is the same as the code name of the layout to which it belongs. The folder usually contains any files used by the CSS classes defined locally in the web part layout's CSS styles property.
Detailed information about the actions available on this tab can be found in the File system dialog help topic.
Please note, only file formats commonly required by CSS stylesheets are displayed here. This includes the following: .css, .skin, .gif, .png, .bmp, .jpg
Additional information can be found in:
|•||Developer's Guide -> Development -> Web parts -> Modifying web parts -> Customizing web part layout|