changing the profile page

I want to make some changes to the profile pages of the users. What are templates/css files I should look at ?

The user profile templates are in templates/user_profile. If you need to change the user profile a lot, it would be better to create a separate application on the side and change askbot/urls.py to disable the built-in profile pages and add to the project main urls.py (not under askbot) so that your custom user profiles are available.

It is also possible take a middle ground approach - use some of the built-in views and add some custom, as well as change styling and some html files.

If you touch only select few templates, then it will be a lot easier to upgrade your site to the newer version of askbot.