i wonder , how can a great project like kde plasma desktop don't have a simple feature like blue-filter (night mode) !

It is available but not installed by default in most cases. I'm not really sure why. Install the plasma-applet-redshift-control package, log out and back in, right click the system tray arrow and choose "Configure System Tray". Scroll down in "Extra Items" and enable "Redshift Control". From there you can use the tray icon to configure it. Hope this helps.

it is now featured by default in plasma 5.17 ,thanks kde