How to modify default login page using theme?
Go to 


  web/sites/default/services.yml


and change the debug value from false to true 

debug: true




You will get file name suggestions like shown in the image, u should use the same names.

Also, was your sub-theme succesfully installed?
Is the sub-theme under this path ? `


  themes/custom/myproject_bootstrap_subtheme


`