Change the theme of IDE between Darcula and Default depends of the hour that you put into the Settings. You can change the hours in Preferences > Other Settings > Theme Switcher. Or you can change the theme Manually in Tool > Change Theme.
Recent change notes
Now the plugin change automatically the theme when the times comes.
General usage instructions
You only need to put you hours in Preferences > Other Settings > Theme Switcher and the plugin do the rest.