Disable rounded corners on cinnamon?

How do you make corners sharp?

If you’re talking about the rounded corners of windows, you can switch to a theme, such as Adpata-Nokto, that uses square corners.

In the Themes app (found in the main menu and also in System Settings), click the Add/Remove tab and install Adapta-Nokto from the list of available themes. Then, back on the Themes tab, click the icon on the Applications line. From the list of installed themes, select Adpata-Nokto. All of your windows will immediately change to square corners.