This theme has bit-rotted because we 3d party developers saw The End coming. This theme will not work beyond Firefox 56. Farewell. BTW--If you want to develop your own theme, these icons are in the public domain.
Version 4 of the Shift Themes. Following a development period of several months, Redshift V4 is designed for use with Firefox 4.0 - 44.* or whenever Mozilla decides to stop changing the version number.