Hello Manthan,
Apparently the datawidget datepicket wants to use a variable that does not exist (anymore) within your styling sheets, my guess it is the $dg-brand-primary that does not exist anymore in your stylesheets add this variable (again) and then it should be good to go, maybe create a branch of an earlier version and try to locate if the variable existed back then and try to locate this variable, afterwards you can add it at the same place.
Hope this helps,
Good luck!