Are there any modules for questions and answers website in Drupal 6? I want it to have theme.
The Answers module should do the trick:
Answers provides a question & answer service. Users can post questions and other users can answer them.
The displays are based on standard CCK node displays and Views, so can be themed in the typical ways.
Some suggestions that might possibly help:
There's a Drupal distribution for it ... (not just a 'module'), i.e ArrayShift. Quote from its project page:
A question/answer site built to emulate the core functionality of the StackOverflow.com site and the StackExchange platform.
There is a Drupal theme for it, i.e the ArrayShift Theme. Here is a screenshot (from its project pages):
Note: Refer to my answer in https://drupal.stackexchange.com/questions/11566/module-for-creating-a-site-similar-to-stack-exchange-sites/154288#154288 for more details about a roadmap for future releases of this ArrayShift.
Disclosure: I'm the (new) maintainer of this distribution,
I hope this does not violate the site's policy on self-promotion.