Have you tried what’s written here : How do I add a custom Taiga theme in a self-hosted Docker container? - Troubleshooting - Taiga Community and here How can I edit the frontend and backend of taiga-docker? - Troubleshooting / Self hosted Taiga - Taiga Community. Hope this help.
I did manage to create a theme using taiga from source code, haven’t tried with a docker installation which seems to be a bit harder, can’t help you more.
