Dark theme or High Contrast mode is mostly used for reading purpose at night or in low light so that the screen light does not affect your eyes. So in Windows, there is an option to change Windows 10 theme into dark mode. By default, it is set to Light theme. You can change it into a dark theme. You can also change Windows 10 style for a particular app into dark mode.
Enable Dark Theme in Windows 10 Settings
First, you need to Open Settings by either using shortcut Windows + I or Click Windows logo and choose Settings
Click Personalization icon
On the left hand side Select Colors
Scroll down Choose default app mode to Dark
Turn on Dark Theme for both Settings and apps
Most of you facing problem to not able to enable dark theme because having an old version of Windows 10 then you can go for either High Contrast Settings or Update Windows 10 to use reader mode. High Contrast also works like Windows dark theme not only in Settings but also Apps.
High Contrast mode in Windows 10
First, you need to Open Settings by either using shortcut Windows + I or Click Windows logo and choose Settings
Click Personalization icon
On the left hand side Select Colors
Right hand side you will see High Contrast Settings under Related Settings
Open High Contrast Settings > Turn On high contrast
In most cases dark themes in file explorer not working in Windows 10. By enabling this High Contrast Mode Settings, File Explorer also will be set as a dark mode.
How to Enable Dark mode for Microsoft Office
You can also set up Microsoft Office in dark mode. Follow below steps
Open Word, Excel which you want to enable dark
Click File on top right corner then, Go to Options
Open General Tab > Color Scheme > Select Black
Press Ok to save your Settings.