Problem with st_widget_set_theme()

[Michele <micxgx gmail com>]

As far as I know, the extension stylesheet has a lower priority than
the default or theme ones. This means that style cannot be easily
overwritten.

I tried to use st_widget_set_theme() in order to: "Overrides the theme
that would be inherited from the actor's parent or the stage with an
entirely new theme (set of stylesheets)" [documentation].

I've created a new St.Theme with swapped application_stylesheet
default_stylesheet

theme = new St.Theme({application_stylesheet: 'path_to_custom_css',
default_staylesheet: 'path_to_default_css'});

and tried to apply it to a St.Box widget

widget.set_theme(theme);

but It doesn't seem to work and I get the following message:

Window manager warning: Log level 8: g_object_ref: assertion
`G_IS_OBJECT (object)'

Any way to get over the problem? Applying the same theme to the stage
seems to work properly instead:

themecontext = St.ThemeContext.get_for_stage(global.stage);
themeContext.set_theme(theme);