Question

This is my website. I have added the categories, however it appears not to be inline. I have tried to style this in a class but this screws up the whole thing.

Do I edit the main style within the themes?

Answer 1

If i unterstand you correctly, just add the following to your themes style.css file. This should do the trick.

.post-categories, .post-categories li {
    display:inline;
    padding:0;
 }

Update:

If you add this style you should see something like that. Is it what you expected? Make sure to clear your browser cache and all your styles are proper loaded.