You will have to configure your i3 to use "XF86MonBrightnessUp" and "XF86MonBrightnessDown" with the appropriate actions that causes your screen to become brighter or less bright. See the i3 FAQ archive.
You will have to configure your i3 to use "XF86MonBrightnessUp" and "XF86MonBrightnessDown" with the appropriate actions that causes your screen to become brighter or less bright. See the i3 FAQ archive.