It's possible that you will need to compile the theme for it to display correctly. We use lessc to help compress the theme assets - we leave these uncompiled in the code repository, but precompile them in the downloadable tarball. You should be able to follow the instructions for our installation from the code repository section
of the official docs to install the dependencies, and then compile the theme using the make command in your Docker environment.
First the dependencies:
sudo apt install npm make
sudo npm install -g "less@<4.0.0" n
sudo n stable
sudo npm install
sudo npm run build
Then the make command to compile each theme's assets. I am not sure if you need to change the path to match how the Docker AtoM env is organized, but here are the commands and paths used in a production installation:
sudo make -C /usr/share/nginx/atom/plugins/arDominionPlugin
sudo make -C /usr/share/nginx/atom/plugins/arArchivesCanadaPlugin
Let us know how it goes!