This is a tool for generating screenshots of GtkSourceView Styles generated with Theme Generator. It is used to populate the images for thief-themes.
The code creates a Gtk.Window with a Gtk.SourceView with a theme rendered. It'll wait for a second for the window to render, then take a screenshot of the window from the screen. It repeats the process until it finishes making screenshots for all the themes in the themes directory.
For some reason, Gtk.OffscreenWindow did not render the syntax highlighting, but is useful in other scenarios.