GtkSourceSchemer is a source code color scheme editor for GtkSourceView applications such as Gedit, Anjuta, and Scribes. It is a standalone Python application, not a plugin. It requires gtk+ 3.x.
For the Gedit plugin version see GeditSchemer.
Place your newly created schemes in ~/.local/share/gtksourceview-3.0/styles/ and relaunch Gedit or the editor that will be using it.