Sorry for the late update on the project, was a little busy writing the code for this project.
Anyways let me give you a brief description of what I have been able to implement so far!
I have added padding to the buttons so that they are aligned in well sepearated rows and columns. We are planning to keep the color of the buttons nearly the same except the buttons for the operators would have a darker shade of grey and the result button would be blue in color. Improving the functionality of Undo buttons has already been implemented by another developer so I just fixed a bug associated with the same. I have also implemented History View which is the most important subtask of this project. The History View stores the expression entered by the user along with the corresponding answer into well defined rows of Gtk.ListBox.You can select and edit the expression in a row by double clicking the corresponding row.
Link to the patch for adding padding to the buttons:
Link to the patch for implementing history view :
Link to image of History View in Gnome-Calculator:
We have more plans for history view and the UI so stay tuned. I would be back with more updates on my project soon!