If you have downloaded off the site. You need to move it into a USB. You create a Folder called PS4 and within that folder you create another folder called THEMES then add the downloaded theme in there. Afterwards connect your usb to the ps4 and go into themes, select custom and it'll ask you where is the theme located. Choose your usb as source and apply your custom theme.
Didnt work for me, I downloaded a dynamic theme from PS4 area, and put it on an USB device, but when i select to put on custom themes, the PS4 dont find any theme on pendrive. All themes from PS4 area are on .p3t format, I searched on Google and its say that is PS3 format. So I dont know what is wrong. I have to Jailbreak my PS4 to install Dynamic Themes from USB?