WIP Add a custom theme

[madmath03]

:construction: Add a custom theme to Taiga front docker image

TODO:

• [ ] Develop a Custom Taiga Dark theme based on main Taiga theme (cf. taigaio/taiga-front#994)
• [ ] Develop Taiga Front Builder Container to build the custom theme
• [ ] Use the Taiga Front Builder Container as multi-stage build and copy custom theme(s)
• [ ] Add the custom theme to the frontend image: https://github.com/Monogramm/taiga-dark