Move ALL extracted folders to your skins folder. The dark skin will not work if the regular skin is not also installed:
\Documents\Sports Interactive\Football Manager 2021\skins\
Run Football Manager 2020 and go to Preferences > Interface.
You should see "FMSCOUT-FM21-Instant-Result” or "FMSCOUT-FM21-Instant-Result-Dark" as an option in the skin drop down of the General options section.
Hit the Confirm button.