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Give the body a background-size of contain.

To center .box on the page using flexbox or grid instead of a margin, add min-height:100vh; display: flex; align-items: center: justify-content: center; or min-height:100vh; display: grid place-items: center to the body.

To center .box on the page using flexbox:
body{
min-height: 100vh;
display: flex;
align-items: center;
justify-content: center;
}

To center .box on the page using grid:
body{
min-height: 100vh;
display: grid;
place-items: center;
}

For a responsive content, replace the width in .box with a max-width and increase the value. max-width: 400px Change the height value to auto.

Use relative units like rem or em as unit for the padding, margin, width values and preferably rem for the font-size values, instead of using px which is an absolute unit. For more on CSS units Click here

Hope am helpful.

Well done for completing this challenge. HAPPY CODING

Good Work, Keep Coding Advice, Use Display Flex