Hi Ryan,
To set a page theme you need to target the page you want to look different.
To do this you need to look at the URL for the particular page or set of pages, and take the information for your CSS selector from that URL.
For example:
Taking the URL from the address bar of my browser window as I am typing this reply, I see the following:
https://moodle.org/mod/forum/post.php?reply=998055#mformforum
From this URL I can see that the page I want to target (should I want to style it) would be #page-mod-forum-post so by adding this ID selector to my CSS stylesheet I can set style that will only target these pages.
Also using FireBug you can inspect the soursecode for your site which allows you to see the extra list of BODY tags for the page you are inspecting.
This is the list I see in this same page:
<body id="page-mod-forum-post" class="format-topics path-mod path-mod-forum gecko dir-ltr lang-en yui-skin-sam yui3-skin-sam moodle-org pagelayout-incourse course-5 context-115 cmid-46 category-1 content-only jsenabled has_dock has_dock_left_vertical dndsupported">
All the above highlighted in purple can be used as class selectors to help you home into a specific page or course or category.
Hope this helps?
Cheers
Mary