Here are five community-favorite themes (available via GitHub or forum searches). Search the exact names below.
RetroArch uses XMB, Ozone, RGUI, and GLUI as its main menu drivers. Custom themes (often called "skins" or "assets") are available for XMB and Ozone. retroarch custom themes download
Then in Settings → User Interface → Appearance, change Menu Color Theme to RetroActive, FlatUI, Neon, MidnightBlue, etc. change Menu Color Theme to RetroActive
