You will need one for each month/theme to be displayed on the top screen of your DS. A theme also includes a background for the bottom screen, three icons for the bottom screen, a background for the file manager's top screen, and a background for the file manager's bottom screen. It helps to use the same three icons and wallpapers for the other three screens, but if you want to have four unique images for each theme, that's 48 pictures to find.;
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Choose your images.
Resize background images (details follow).

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Choose and resize icons (details follow).

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Build a theme (details follow).

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Build the rest of the themes.

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Install themes (details follow).

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Enjoy your personalized DS!
{"src": "wikihow-all_test", "title": "How to Theme an R4DS Menu"}