First, get the bundle identifier for the app that you want to turn off dark mode.

osascript -e 'id of app "{app name}"'

# e.g.
osascript -e 'id of app "Dash"' # com.kapeli.dashdoc
osascript -e 'id of app "Chrome"' # com.google.Chrome

Then run

defaults write {package.name} NSRequiresAquaSystemAppearance -bool Yes

# e.g.
defaults write com.kapeli.dashdoc NSRequiresAquaSystemAppearance -bool Yes
defaults write com.google.Chrome NSRequiresAquaSystemAppearance -bool Yes

If you wish to restore the app’s theme to its default configuration then run the below command. Doing so will re-enable the dark mode for the particular app.

defaults write {package.name} NSRequiresAquaSystemAppearance -bool no

# e.g.
defaults write com.kapeli.dashdoc NSRequiresAquaSystemAppearance -bool no
defaults write com.google.Chrome NSRequiresAquaSystemAppearance -bool no

# or
defaults delete com.google.Chrome NSRequiresAquaSystemAppearance

Alternatively, use the Nightowl app to have this control in an easier way