fix: default dark when plotting

src/pycaputo/__init__.py

@@ -268,7 +268,7 @@ def fracplot(
  dim = sol.y.shape[0]
  overrides = {"lines": {"linewidth": 1}} if dim == 3 else {}
 
- suffixes = _get_default_dark(default=dark)
+ suffixes = _get_default_dark(default=bool(dark)) if dark is None else ((dark, ""),)
  outfile = None
 
  from pycaputo.utils import figure, set_recommended_matplotlib