Im not sure about boundaries but if you want to modify certain specific places like the Header then you just modify it under the Header section only. Index if im not mistaken is your structural look. So if you were to modify that then it will change the look of any page you go to. For example if you were to put ‘x’ on your index file then page 1 will have ‘x’ on it, and all of the other pages as well.
Hope this helps.
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