By default, Core uses the income and expense accounts set in Global Settings > Accounting > Default Accounts > System Accounts for all your activity and expense items. However, you can change these per item in the Activity Items or Expense Items screen. For that you need to make the Income Account and Expense Account columns visible on these screens by clicking More > Show/Hide Columns. Then select the relevant income and expense accounts to be used for the items from the drop-downs. Check Core Help for details.