self.theme_settings = registry.forInterface(IThemeSettings, False)and later used in the view template like this:
For anyone interested, my image scaling branches are https://github.com/plone/plone.namedfile/tree/datakurre-image-scaling-queue and https://github.com/plone/plone.formwidget.namedfile/tree/datakurre-image-scaling-queue
Right now the feature is on by default. Just using those branches is enough. That may change during today once I try to polish and test it and write a PLIP.