An automatic page but with CSS or additions

Some pages come standard. Archive, photos, etc.

In the past, I managed to mix my own CSS code with a photo page, but now, with the new theme, I can’t figure out what I did. I had a photos page, and I added CSS code to increase its width (so the photo page will be wider, but only the photo page).

Another thing: I want to nest a couple of custom pages under Archive. For example, I have “old blog” and “wiki,” both are custom pages; I want these both to show on top of the posts that already exist in the Archive page. How would I go about adding the text (HTML) and append that to the existing Archive page?

I’m guessing I could figure out what the Hugo template is for the archive page and create the whole. page by hand that is, custom-made from the template with the additions I want built in… but is that the only way to do it?