The theme needs spicetify in order to be installed.
Just run these commands on a terminal:
git clone https://github.com/santoru/Arc-Dark-Spotify-Theme
mv Arc-Dark-Spotify-Theme ~/.config/spicetify/Themes/
spicetify config current_theme Arc-Dark-Spotify-Theme
spicetify apply
Highly inspired by arc-theme