Changes made:

Accounts.cls
- changed "onAfterInsert" method to call on the Accounts domain via the domain factory and the Opportunities domain will work only with the "IAccounts" interface.
- Used the "Application.Domain.newInstance(Set<ID>)" method to automatically select the records verses using an inline query.
- Now working with the "fflib_ISObjectUnitOfWork" interface

Application.cls
- Added entry for Account.SObjectType to the SelectorFactory call to all above changes to Accounts domain to work correctly.

Author: John M. Daniel

sfdx-source/apex-common-samplecode/main/classes/Application.cls

@@ -52,6 +52,7 @@ public class Application
 	public static final fflib_Application.SelectorFactory Selector = 
 		new fflib_Application.SelectorFactory(
 			new Map<SObjectType, Type> {
+					Account.SObjectType => AccountsSelector.class,
 					Opportunity.SObjectType => OpportunitiesSelector.class,
 					OpportunityLineItem.SObjectType => OpportunityLineItemsSelector.class,
 					PricebookEntry.SObjectType => PricebookEntriesSelector.class,

sfdx-source/apex-common-samplecode/main/classes/domains/Opportunities.cls

@@ -103,16 +103,27 @@ public class Opportunities extends fflib_SObjectDomain
 	public override void onAfterInsert()
 	{
 		// Related Accounts
-		List<Id> accountIds = new List<Id>();
-		for(Opportunity opp : (List<Opportunity>) Records)
-			if(opp.AccountId!=null)
+		Set<Id> accountIds = new Set<Id>();
+		for(Opportunity opp : (List<Opportunity>) this.records)
+		{
+			if( opp.AccountId != null)
+			{
 				accountIds.add(opp.AccountId);
-				
-		// Update last Opportunity activity on the related Accounts (via the Accounts domain class)
-		fflib_SObjectUnitOfWork uow = new fflib_SObjectUnitOfWork(new Schema.SObjectType[] { Account.SObjectType }); 
-		Accounts accounts = new Accounts([select Id, Name from Account where id in :accountIds]);
-		accounts.updateOpportunityActivity(uow);
-		uow.commitWork();				
+			}
+		}
+		
+		if ( ! accountIds.isEmpty() ) 
+		{
+			// Update last Opportunity activity on the related Accounts (via the Accounts domain class)
+			fflib_ISObjectUnitOfWork uow = new fflib_SObjectUnitOfWork(new Schema.SObjectType[] { Account.SObjectType }); 
+		
+			
+			IAccounts accounts = (IAccounts)Application.Domain.newInstance( accountIds );
+		
+			accounts.updateOpportunityActivity(uow);
+			
+			uow.commitWork();				
+		}
 	}
 
 	public void applyDiscount(Decimal discountPercentage, fflib_ISObjectUnitOfWork uow)