These forums have dedicated threads for theme customization.
RetroArch does not support "one-click" theme installation in the same way commercial software does. Customization requires the user to manually manage assets in the file system. However, the flexibility of the and XMB drivers allows for deep aesthetic customization ranging from minimalist text to full console-accurate UI replicas. retroarch custom themes download
If you downloaded a custom Menu Driver (a .dll on Windows or .so on Linux): These forums have dedicated threads for theme customization
The classic PlayStation-style horizontal menu. This is the most "themeable" driver. retroarch custom themes download
If you're interested in trying out RetroArch custom themes, we recommend: