Hi there!
Firstly, that is not the official way to install Taiga with Docker, so I’m not familiar with the peculiarities of that particular repository. That said, it seems they use the official docker images, so the steps should be similar enough.
As mentioned here, you need to compile your own version of taiga-front with the theme you want to change.
Once that is done, you would need to build a docker image and use that instead of the taiga-front image.
I hope this helps.
Best regards!
