Fork of the theme Classic Compact by Ken Barbalace for Pale Moon.
Simply download the contents of the "src" folder and pack the contents of both the "options" and "theme" folders into a .zip file each. Then, rename the files to .xpi. Pack both of these XPI files with the install.rdf file in the "src" folder into another .xpi. Then, drag into the browser. You can install these separately if you wish, however it is strongly advised to install together.
You can grab the latest release either from the Releases section of this repository, or the Pale Moon Add-Ons Site.