I installed a theme on my computer but it does not work properly, Please check with below attachment css return 304.
304 is not modified.
A 304 Not Modified message is an HTTP response status code indicating that the requested resource has not been modified since the previous transmission, so there is no need to retransmit the requested resource to the client.
So you've got a look at the steps you are following in terms of modifying the theme css.
Magento is not detecting a change.
Or do you mean you have installed a theme and it's not showing.
Instructions on how to apply a theme are here: