SQL Server management studio does not have the dark theme by default. You can make the below change to enable the dark theme option.

1. Go to the below path
   C:\Program Files (x86)\Microsoft SQL Server Management Studio 18\Common7\IDE\
2. Modify the below file in Administrator mode. If you are using notepad++, you can launch it in admin mode.
   File name: ssms.pkgundef
   
3. Search for Dark and add // in front of the below line to comment and save the file.
   [$RootKey$\Themes{1ded0138-47ce-435e-84ef-9ec1f439b749}]
   
4. Open SSMS => Tools => Options => Environment => General => Color theme and choose dark theme.
   
5. Restart the SSMS.
   

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