[Django]-Overwrite an Existing Template Tag method in Django 1.8

I’m afraid that this kind of change can not be done easily. To overwrite this method you would need to modify Django source code. It’s not recommended though.

If it’s only CSS/HTML change, you can do it by adding the style with JavaScript. There is a simple way to add the custom JS to the admin by creating custom form and adding new JS files to Meta class.

Have a look on the docs and example here.