The Potlatch 2 styles need to be patched in order to work with the JOSM MapCSS implementation.

Potlatch 2's repository is git://git.openstreetmap.org/potlatch2.git

 * nodist/styles/potlatch2/source
        contains the unmodified source files copied from git.openstreetmap.org/potlatch2.git/history/master:/resources/stylesheets

 * nodist/styles/potlatch2/patched
        contains the manually edited version

 * resources/images/icons
        copy of git.openstreetmap.org/potlatch2.git/tree/HEAD:/resources/icons

 * resources/styles/standard/potlatch2.mapcss
        concatination of the patched files; the ant task 'assemble' helps with this (use build.xml in the current folder)

When updating from upstream, keep all 4 locations in sync.

At time of writing, the last update used 28c84372002671c8d71a81a2e423f4965388edd7 (2019-06-21) but better check the commit dates as well.