Enabling the dark theme:

Open a terminal and run:

git clone https://github.com/veyselerden/elementaryos-dark-theme-enabler-script.git && cd elementaryos-dark-theme-enabler-script && chmod +x enable-dark-theme.sh && ./enable-dark-theme.sh

Disabling the dark theme:

Also open a terminal in the folder and run:

cd elementaryos-dark-theme-enabler-script && chmod +x disable-dark-theme.sh && ./disable-dark-theme.sh