結論

アプリケーションの中身を掘っていって、Pixelorama ファイルに実行権限を付ける。

参考

公式のダウンロードページに、そのままの記述があります*1。

If you're using MacOS, you may encounter some errors saying that Pixelorama is damaged. This is because the latest versions of MacOS mark the launcher of unknown binaries as a non-executale file. After you extract the .zip file, right-click on the .app file > show package contents, then open a terminal inside its Contents/MacOS folder. Then, type chmod +x Pixelorama . Go back to the .app file and you can now open it normally. Special thanks to @rafaelgdp for finding the solution!