How can i edit the code behind a appian project. Is it possible to get the project opened in a IDE and customize it as we wish

While many portions of the Appian platform are based on Java, modifying the base product code is not supported in anyway. It can also lead to you not being able to upgrade in the future or get assistance from Appian's support team as they will no longer be able to support base product functionality under a customized environment. 

Back to your task question, I suggest you look through some of the documentation on how task assignment works within Appian. When assigning a task to a group of users, the individual who will work that task must accept it, thus taking the task out of everyone else's queue. If you are assigning a task to an individual, they will not see the dialog as they are the only user that can access that task.

https://docs.appian.com/suite/help/20.1/Tasks.html

But can we not just let anyone submit the task without asking them if u accept it or not. I understand thats what takes it away from everyone else's queue. But can we not update other's queue once they are submitted . 

So, its not possible to modify that flow when have more than one user assigned to a particular task?

And about the base product code, even if we cannot change the base code , is it possible to add styles or change the code for a form that come with appian. 

I am kind of new , so it will be nice if redirect me to a page where i can find ways to customize layouts , and the functionalities and add styles.

The accept button helps to prevent a race condition in the scenario of more than one user trying to complete a task and submit it at the same time. You may be thinking about smaller low risk tasks, but it functions the same in Appian regardless.

You can't modify CSS, but there are ways you can modify the look and feel of an application or interface within the base product. Take a look at the UX Design Guide for some additional information on best practices and ideas on UX things that can be tweaked. If you haven't already started working through the online training videos, it may be beneficial to look at some of the training on interfaces and SAIL. 

https://docs.appian.com/suite/help/20.1/ux_getting_started.html

Hi NARESH KUMAR - 

ericg329 Is providing sound advice.  

Consider the following:

1. You could set the assignment variable of the task to a user in a group before the task loads. 
   When a task is assigned to one and only one user, the accept header isn't shown.  
   1. This would depend on expressions or rules to facilitate "round robin" or some other fairness assignment mechanism. 
      (In my opinion, this design convention is almost always more trouble than it's worth, so I always recommend against this)
   2. Assigning tasks to users and not groups is often not recommended because under this condition, task processing can be delayed if an individual is not available to process their tasks.
   3. You can add an exception timer to a task assigned to an individual so that if it does not get completed within a prescribed time, it is released to flow out and can then be reassigned systematically.
      
      
2. Assignment to groups promotes effective task processing - 
   1. What is the negative impact of the acceptance banner?

Hi NARESH KUMAR - 

ericg329 Is providing sound advice.  

Consider the following:

1. You could set the assignment variable of the task to a user in a group before the task loads. 
   When a task is assigned to one and only one user, the accept header isn't shown.  
   1. This would depend on expressions or rules to facilitate "round robin" or some other fairness assignment mechanism. 
      (In my opinion, this design convention is almost always more trouble than it's worth, so I always recommend against this)
   2. Assigning tasks to users and not groups is often not recommended because under this condition, task processing can be delayed if an individual is not available to process their tasks.
   3. You can add an exception timer to a task assigned to an individual so that if it does not get completed within a prescribed time, it is released to flow out and can then be reassigned systematically.
      
      
2. Assignment to groups promotes effective task processing - 
   1. What is the negative impact of the acceptance banner?