The directory graphics contains a script that converts all aseprite files into C code when run with the -a option. These files are not uploaded so run the script without this option in order to convert the .png files to C code.
cd graphics ./convert_graphics.sh
Install platformio as described here
plugin your Arduboy and run:
platformio run --target upload
If this is not working then push the reset button of the Arduboy and try again. Alternatively hold the UP key then switch on the Arduboy. Now you can run the platformio command from above.