Color problem on Kubuntu 16.04 - themes aren't applied

Solution 1:

I'm guessing you've installed some other GTK based Desktop Environments (Such as Lubuntu or Xubuntu) in your Kubuntu installation. Though the theme changes, they revert back to the ugly Striped look after a reboot. The solution is installing gtk3-engines-breeze.

sudo apt-get install gtk3-engines-breeze kde-config-gtk-style

After installing, Open System Settings -> Application Style -> Gnome Application Style and Change the theme for both Gtk2 and Gtk3 to Breeze Dark.

If the theme doesn't change immediately after applying:

If all the theme component doesn't change to Breeze Dark immediately, Go to Settings -> Workspace Theme and select Breeze Theme by clicking it, this will activate the Apply button. Now, change the selection back to Breeze Dark Theme by clicking it and finally Click the Apply Button.