If classic or tango color scheme, automatically switch to oblivion when switching to a dark GTK theme

If classic or tango color scheme, automatically switch to oblivion when switching to a dark GTK theme.

And vice versa (to the previous setting).

We know that classic and tango have light colors, and that oblivion has dark colors, so when the user switches the GTK theme to a dark/light theme, gedit could also switch its color scheme (or "style scheme").