Your config.toml file needs to go in a .streamlit folder in the root of your project directory. You just manually create it if you don’t already have one. For example:
your-repository/
├── .streamlit/
│ └── config.toml
├── your_app.py
└── requirements.txt
However, the current support for font in config.toml is:
# Font family for all text in the app, except code blocks. One of "sans serif",
# "serif", or "monospace".
# font =
If you are looking to use a different font than the three available, you have to override it with CSS.