Hello @PRAGHATIESH S 

1. Navigate to All in the app navigator and search for sys_properties.list.
2. In the list search bar, search for glide.ui.polaris.dark_themes_enabled.
3. Set glide.ui.polaris.dark_themes_enabled to True to enable the dark theme in all of Next Experience.
4. In the list search bar, search for glide.ui.polaris.core_ui_exclude_dark_mode.
5. Set glide.ui.polaris.core_ui_exclude_dark_mode to True to exclude the dark theme in classic environment.

Plz mark my solution as Accept, If you find it helpful.

Regards,

Samaksh