How to change the background of all pages of the website to the same?

How to change the background of all pages of the website to the same? That is to change the global background of the website to the same, the operation method is as follows:

Click \"Theme\"---\"\"Change background\"---\"Here you can choose the background color, or set the background picture below, click \"Custom\"--->>\"Upload \"---》Select \"Whether to scroll with the webpage\", select \"Tiling mode\", \"Tiling location\", \"Background size\", after setting, click \"Save\". That is, it is saved to the background of the website. If you want to display it in the front of the website, you need to click the \"Publish\" button in the upper right corner of the page.
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