How to disable Mac's radius corner of the window

Update on 2020-12-30: Use this way to modify system files on Big Sur!

Update for big sur! (2020-12-07)

The big sur doesn't allow making root volume writable anymore, since it's a snapshot now.

Follow these instructions to modify system files (or see details here):

The WindowFrame_WindowShapeEdges is not in the SystemAppearance.car (DarkSystemAppearance.car for the dark theme) anymore but in the Aqua.car (DarkAqua.car for the dark theme).
The old ThemeEngine (v2) doesn't support the big sur anymore, you need to use the v3 branch released by @jslegendre.

Below is the original post:

Tested on Catalina 10.15.3 | Last Updated: Wed Apr 8 22:32:32 CST 2020

Disable SIP (There are lots tutorial online to teach you how to do it, will not expand the details here)
Install ThemeEngine (Note: only this build supports catalina 10.15)
Make system volume writable: sudo mount -uw /
Backup original theme resources: rsync -rI /System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/ .
Create a new folder for editing the theme file: mkdir edited && cd edited
Copy a theme file here and edit it (SystemApperance.car for the light theme and DarkAquaAppearance.car for the dark theme). Take SystemAppearance.car as an example:
1
2
cp ../SystemAppearance.car .
open . # Open this folder with finder
Double click the file SystemAppearance.car to edit it with ThemeEngine that we installed before
Edit WindowFrame_WindowShapeEdges
- Replace masks to squares
- Replace base images to pure transparent images

Save all changes, replace the modified SystemAppearance.car back to the system path.

1	sudo cp SystemAppearance.car /System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/

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