MacOS – Can I change 10.8.2 Mountain Lion window corners from rounded to squared

Tested on 10.7.3 & 10.7.4

To turn rounded corners off for all windows use the following command in Terminal.

defaults write -g AppleUseCoreUI -bool false ; killall Finder

To restore the rounded corners for all windows use the following command in Terminal.

defaults write -g AppleUseCoreUI -bool true ; killall Finder

NOTE: For this to take effect on already open applications those applications must be closed and reopened (this is why the commands above include killall Finder). Some Applications may interact differently once CoreUI is turned off.

ALSO: This does not affect the corners at the bottom of the window. They will still appear rounded. (As noted in comments by Daniel Lawson)

If you are not comfortable with the killall Finder being used in your command the steps can be taken to ensure the same result.

Step 1. (Turn off rounded corners)

• Run the following Command

  defaults write -g AppleUseCoreUI -bool false
  

• Remember to close and reopen any Applications that were open when prior to this command being run.

Step 2. (Turn off rounded corners)

• Refresh Finder

  Key combo command + option + esc... Choose Finder and click Relaunch

• Remember to close and reopen any Applications that were open when prior to this command being run.

To restore the original rounded corners, simply use the same 2 Steps and replace the false found at the end of the command with true as shown in the following command.

defaults write -g AppleUseCoreUI -bool true

Remember: It is very likely that as you update your OS the rounded corners will reappear. However, even after your next OS update, the same steps listed above are likely to function the same. :–)

This one works well for me: toggle-osx-shadows.

It is easy to compile and use, and there are only 17 lines of code.