This is an attempt to get a working Crystal icon theme for KDE Plasma 5, by combining/mixing different versions of the Crystal icon theme (that were created for KDE 3 desktop). Personally, I think Crystal is the best ever icon theme for the Linux desktop, and I'd like to be able to continue using it on my KDE Plasma 5 desktop.
Original author: Everaldo Coelho (https://en.wikipedia.org/wiki/Everaldo_Coelho).
Adapted for KDE Plasma 5 by: Viet Dang (dangvd at gmail.com).
License: The original Crystal icon sets were licensed under the LGPL.
To install, unzip the .tar.gz file then copy the extracted Crystal Remix folder (e.g. crystal_remix-1.0-beta1 folder) to the system's icon themes folder (e.g. /usr/share/icons).
Note: you might have to refresh KDE Plasma 5 icon cache (e.g. running '$ rm ~/.cache/icon-cache.kcache') when you switch to using the new icon theme for the applications to refresh their icons.
The original posts from Everaldo are below, although the download links are no longer working:
However, the different Crystal icon sets can still be found online, for example:
Crystal Project icon set:
Crystal SVG icon set:
Crystal Clear icon set: