feat: allow to add custom style classes

# Description

During the rendering process, if we set `__style__=True`, then the style classes are automatically generated during the build. It would be interesting to allow the user to include its own classes. 

# Idea

To use a syntax like:

```python
from app import config

config.custom_style = {
    "some_style": {
        "aliases": [],
        "style": ""
    }
}
```
