If you discover that another academic has written a thesis on a topic similar to your own, don't stress out. Carefully read the material to gain an understanding of what it is that the prior thesis has accomplished and consider ways your thesis might further develop the topic or might approach the topic from a totally different perspective. You are likely to find that the former thesis is not the same as the one you are considering after all. To be sure, show the competing thesis to your advisor; he or she can give you indispensable advice. If you discover that another graduate student is writing a thesis on the same topic you've chosen, you could also consider contacting that author to get an even better idea of whether your ideas overlap. (Be careful not to give away too much of your own thinking on the topic as you conduct this discussion.)
So, if you want to make the Category whose ID number is 6 look different from what it is currently (and different from other Category pages), you would want to create a category- file. If you want to make all Category pages look different from other archive pages (such as date and author archives), then you would want to create or modify the file. If you want to make changes to the look of all archive pages, you can create or modify the file. And if you modify the file, you will affect your entire blog.