You can adjust the grid line color using the CSS theme. It's defined using these classes:

*_matrix_horizontal_line
*_matrix_vertical_line

See also:

Availability

Availability of this feature in DayPilot editions:

LitePro
DayPilot for JavaScriptFull SupportFull Support
DayPilot for ASP.NET WebFormsFull SupportFull Support
DayPilot for ASP.NET MVCNo SupportFull Support
DayPilot for JavaNo SupportFull Support