Lumo defines a set of CSS custom properties you can use to apply consistent visual styles across your application.
Import style.html to use them.
You can adjust the top level border radius, which affects many element themes directly.
If you want to set the border radius to zero, you need to specify a unit for it as well (i.e. 0px or 0em), as it can be used in calc functions which will be undefined if --lumo-border-radius is a unitless value.