Installing themes properly is important to making sure they can be updated by Powercord's updater. The following command will work if you installed Powercord to the default location. Open a command line and run the following command:
cd powercord/src/Powercord/themes && git clone https://github.com/overheremedia/Indicord
Then, reload Discord Canary using ctrl+r (or cmd+r for mac) and your theme should start working.