GTK+ Theme Changing in Gazpacho
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Introduction
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The Hildon widgets looks and behave a little bit weird in a plain GTK+
environment so in the Maemo platform a custom GTK+ Theme is provided to
add some branding and make a difference when running in the Maemo devices.

On the other side Gazpacho GUI is designed to work using standard GTK+
themes and when running using the Maemo GTK+ theme it looks and behaves wrong.


Goal
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* To run Gazpacho using two different GTK+ themes: one for the Gazpacho GUI
  and one for the widgets used in Hildon projects where a Hildon project is
  a Gazpacho project that uses Hildon widgets. Usually theses two themes
  would be the standard GTK+ one for Gazpacho GUI and the Maemo Seapwood
  theme for the Hildon projects.


GTK+ support for changing the theme
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Right now GTK+ has support for loading a different set of GtkRC files and use
them instead of the boring system default.

You usually do that with the following (Python) APIs:

 gtk.rc_set_default_files([rc_filename])
 gtk.rc_reparse_all_for_settings(yourwidget.get_settings(), True)


The problem
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The problem is that when you change the rc files and reparse them you are
changing the theme for *all* your widgets in the application. There is no way
to programatically do this only for a specific set of widgets. One of the
reasons for this is that all the widgets in a GdkScreen share the same
GtkSettings.


The solutions
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Solution A: Fix GTK+

It seems that the mechanism to set a rc file per widget is already there in
GTK+ but it is private/internal and it is not exported as a public API.

When you call gtk.rc_reparse_all_for_settings you are basically creating a
bunch of GtkStyles and setting them to all the widgets in your app.

We would need just a hook to select the set of widgets we want to affect with
this change in the theme.

Problems:

  - Even when this looks like a simple solution the integration in a complex
    library like GTK+ could be non trivial and it may break other apps.

  - Inclusion in GTK+ upstream could be hard because of the previous point.


Solution B: Change the RC Files on-the-fly

A rc file is a set of style rules for GTK+ widgets. Each rules is targeted to
a widget or class of widgets. Maybe it could be possible to change the rc file
just before the call to gtk.rc_set_default_files and change its target
descriptors to use the widgets we want to affect.

Problems:

  - A rc files has a non trivial syntax that would need to be parsed. GTK+ is
    already parsing it so we would be duplicating code.

  - There are serveral ways to specify a target in a rc file so we should be
    careful in the way we modify the rc file on-the-fly

  - If we change the name of a widget in Gazpacho the rc file would need to be
    changed again and reloaded.