The theme is called “wporg-developer” and is based on underscores.
It is not available to download as-is, but it of course open source and GPL licensed. You can get the whole source from the wordpress-meta environment ; you’ll need to use VVV to install the whole thing locally on your computer.
Though this theme has been made specifically for wordpress.org so I’m not sure it will be 100% ready to use as-is on your own site, but you can certainly use it as a starting point to create your own version.