It affects to all corners of C1.Win.Accordion.C1AccordionPage headers when C1.Win.Accordion.C1AccordionPage is collapsed. It affects to outer corners of C1.Win.Accordion.C1AccordionPage headers and expanded page area when C1.Win.Accordion.C1AccordionPage is expanded.
It is themeable property, use C1Accordion\Pages\Corners node to customize it with c1theme.
C1Accordion c1Accordion = new C1Accordion();
c1Accordion.Styles.Pages.Corners = 4;