Sony did not block custom static themes, but some dynamic themes require a signed .edat file. If your installed theme shows a black screen, it’s a corrupted or unsigned dynamic theme. Stick to .p3t files for guaranteed compatibility on official firmware (OFW).
For a wider variety of "unofficial" or fan-made dynamic themes, community repositories are the primary source: How to get Dynamic Themes on PS3 ps3 dynamic themes download 2021
Since "2021" was the tail end of official support, you won't find new official themes released after that date, but you can still access the massive backlog of official Dynamic Themes. Sony did not block custom static themes, but