The Nioh Bloody Grave Dynamic Theme was a pre-order bonus for players who pre-ordered Nioh from Amazon (and potentially other retailers; apparently you also got it if you pre-ordered the standard or deluxe edition from the PlayStation Store).
Here is a video of what the Nioh Bloody Grave Dynamic Theme looks like on the PS4:
How to Get the Nioh Bloody Grave Dynamic Theme
If you pre-ordered Nioh as a physical disk from Amazon or another retailer, you should have already received a code (as is common with many pre-order bonuses) that can be used in the PlayStation Store to redeem the digital item.
Once you redeem the code, you should be able to download the theme, if it doesn’t start downloading automatically. After the theme has downloaded (it shouldn’t take long over a good Internet connection, as it is a relatively small file) you can activate it as shown in the video above:
From your main PS4 menu, go to “Settings”, then “Themes” and “Select Theme”. This should show you all the available themes for your system. If you don’t have any others downloaded, then you will probably just see the Nioh theme. Press “X” to choose the theme and then click “Select” to activate it. Then when you go back to the main menu, you should see the new theme in your background as you navigate around your normal PlayStation menus.
“Can I Still Get the Theme if I Didn’t Pre-Order the Game?”
If you didn’t pre-order Nioh, it doesn’t look like there is a way to get the theme, even if you are willing to pay for it. Unfortunately some pre-order bonuses are like that. This is one of many reasons why it’s a good idea to pre-order video games, especially ones from Amazon, and especially if the bonuses are cool.