You could extract them from any of the builds on the Libretro server. But that’s probably more painful than simply downloading the ZIP file from the GitHub RetroArch Assets repository. You can find the PNG files in the /xmb directory and the SVG files in the /src/xmb directory.
GitHub is where we store all the source code for the project, including all the icons.
I just wanted to let you know that NeoActive and RetroActive themes have officially been retired from Retroarch. RetroActive has been merged with FlatUI to become the FlatUX theme, which should be available in an upcoming release.
If you still want to use these themes, they are archived over on baxy-retroarch-themes. You can download them and install them as a custom theme.
If you want to take over development for Retroactive or Neoactive, you are welcome to fork the repo and start contributing.