If you want to enable dark theme (dark mode) in Windows 10, now Windows offers a setting for it. After enabling dark theme, Windows Settings Pages and File Explorer turns dark. Also some applications those you download from Windows Store also turn dark. But it’s up to application developer to support Dark Mode and unfortunately many do not.
To enable Dark Mode (Dark Theme) in Windows 10, launch main “Settings” section of Windows. Then head to Personalization > Colors. In the “Colors” settings page just scroll down a bit and you’ll see “Choose your app mode” title. Select the “Dark” option under this title and everything turns to dark now.
Switching to dark theme will save your laptop’s charge and also gives less harm to your eyes while working at night.
To reduce the space taken by Cortana, right-click on an empty part of the taskbar, select Cortana and choose option what you want.