Consider how most Pages on your Site should look (Alignment, Width, Padding, Text Alignment, Fonts, etc...), and make these changes in the Design tab.
Then, if needed, you can make more specific adjustments to individual Pages in the Content tab within the Design menu of each page. There you can override the same controls on the global Design tab, and fine-tune additional controls for that Page only. All new Pages will start with the settings as defined in the Design tab.
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