flat.icns is a set of flat icons for OS X for a more uniform dock.If you want to change original icons with flat style icons then you can use these icons.
You ca use Litecon (app which allows you to change your system icons quickly and easily)
Just drag and drop the icons onto their corresponding applications, and then log out and back in to refresh the dock.
Some programs may require manual installation (for example, if they are not directly in the Applications folder). The icns folder contains all the .icns files for this.
– Find the .icns file for the program
– Navigate to the program
– Right-click on the program and select ‘Get Info’
– Drag the .icns file onto the existing icon for the program in the info panel