I've never seen a reference so I don't think one exists, but I may be wrong.. Using the "Search templates" feature in the left column is a helpful way of finding a template. Simply paste some unique code into that area and it should narrow down results for you
For example, the editor which I'm currently typing in is wrapped with a css class called
message--quickReply. When I search for that piece of code, the
quick_reply_macros template file is returned, which contains the HTML for this block, as well as the
message.less file which contains the css portion.
If possible, try doing most of your modifications with css/less (place your code into the extra.less file) and try to limit the amount of HTML modifications in your theme. The main HTML file which you'll likely want to edit is
PAGE_CONTAINER, which controls the header, footer, navigation, wrapper, etc. Keeping HTML modifications to a minimum is a nice way of ensuring your theme automatically updates between XF versions, and it also helps in maintaining compatibility with 3rd party addons.
To give an example of what's possible with this technique:
all of my themes only have one modified HTML file which is PAGE_CONTAINER. All other style modifications are controlled by css.