Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Stroke
AI_ART_TAG_STROKEDParameter: Stroke Color
AI_ART_TAG_STROKECOLORParameter: Stroke Width
AI_ART_TAG_STROKEWIDTHParameter: Fill
AI_ART_TAG_FILLEDParameter: Fill Color
AI_ART_TAG_FILLCOLOR