What's the problem this feature will solve?
Currently, users cannot control whether markers in the Altair visualization backend are filled or just outlines. The filled status is hardcoded to True in mesa/visualization/backends/altair_backend.py, limiting customization options for agent portrayals. A FIXME comment in the code specifically identifies this limitation:

# FIXME: Make filled user-controllable
filled_value = True

Proposed Solution
I propose updating the AgentPortrayalStyle component to include a filled attribute (defaulting to True to preserve existing behavior). The AltairBackend should then be updated to respect this attribute, allowing users to define agents with unfilled/outline-only markers by setting filled=False in their portrayal.

Additional context
I have already implemented this locally and am preparing a Pull Request to resolve the FIXME.