How do I remove rounded borders in GNOME Shell?

From the ArchLinux Forum:

edit the css in /usr/share/gnome-shell/theme/gnome-shell.css 
(the .panel-corner properties).

The credit goes to the forum user "gimmy". Refer to the original forum in the sources below if you would like to view the entire thread.

Source: ArchLinux Forum - https://bbs.archlinux.org/viewtopic.php?id=138403

For the people who use a custom theme: A better way is to modify the gnome-shell css file of the theme. If you use a gnome-shell.css file with your custom theme, edit the file with a text editor and just add these lines:

.panel-corner { 
 -panel-corner-radius: 0px;
 -panel-corner-background-color: transparent;
 -panel-corner-border-width: 0px;
 -panel-corner-border-color: transparent;
}

"Remove Rounded Corners extension" is a good extension but sometimes it bugs with the "User Theme" extension...

Any easier way to remove rounded corners and have it persist even after theme updates is to use the Remove Rounded Corners extension.

Remove Rounded Corners Extension