Does this help?:

How to Enable Dark Mode on a Chromebook (howtogeek.com)

First, restart the system and then open the Quick Settings by clicking on the time at the bottom right corner of your screen. Click the upward arrow to expand it if the menu is collapsed. There will be a Dark theme toggle there now, which . TOS76> can click to enable and disable dark mode on your Chromebook. It is pretty simple to solve this issue. 

Turn on dark theme

1. Open your device's Settings app 
2. Select Accessibility.
3. Under "Display," turn on Dark theme.

See more. 

@keval22 To enable dark mode, navigate to Settings > Personalization > Colors, then open the drop-down menu for "Choose your color" and pick Dark. Dark (and Light) mode change the look  of the Windows Start menu and built-in apps.

@headgreenThanks for sharing its working.

@mickymar232 
Thanks for sharing.!!
Did you know any shortcut in Keyboard in windows for unable the dark mode.

To enable dark mode on your Chromebook, go to Settings > Personalization > Set your theme > Dark, or click the clock > Settings > Appearance > Dark theme.