I could be wrong, but there are three ways to insert the body code:
1. Use the embed code element in the page you want to insert that code. It will be automatically inserted before the body tag. For that page only.
This will not change your theme, and you can still get support.
2. Go to themes-->html/css editor. Select the page type you have. Go all the way down and insert your code just before </body> tag.
Remember that this code will be there for ALL the pages with that page type.
3. If you just want to use the body code selected pages that use the same header type. Go to themes-->html/css editor. select the page type. Copy all the codes in between <html> and </body> tag.
create a new page type - give it whatever name you want like junk.html. Delete everything in between <html> and </body> tag. Paste the code you have copied, and insert your new code just above the </body> tag.
In future if you want to create new pages with the same code just use junk.html as your page type.
# 2 and # 3 will change your theme name, it COULD be an issue if you wanted to change the templates or get support in future.
You can try it out. One thing I have noticed with html/css is that if stuff works it works. If it does not work you will have blank space. I would advise you to create a new dummy site before trying this and experiment on that dummy site before going to live site.
