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This folder contains CSS files that control the apperance of serveral UI components e.g. the menu. Each CSS file is named ControlType.Metro.css. So if you want to change the apperance of the menu you should edit the file Menu.Metro.css
telerik/metro/xxxxx
This folder contains other resources that the telerik controls depend on, each control has a sub folder with the name ControlType so if you want to edit any images used in the menu go to the subfolder Menu and make your changes to the image there.