Fork of the theme "Walnut for Firefox" by Alfred Kayser for Pale Moon.
Simply download the contents of the "src" folder and pack the contents into a .zip file. Then, rename the file to .xpi and drag into the browser.
On Unix systems (or Windows 10, with WSL) you can optionally run build.sh instead. Running this as-is will produce a .xpi file ending in -dev, and if run from the command line and appending a number (e.g. ./build.sh 2) will append that number to the filename instead.
You can grab the latest release either from the Releases section of this repository, or the Pale Moon Add-Ons Site.