Add possibility to disable backdrop

Submitted by Yeti

Description

Backdrop makes difficult to read text (labels) in windows that do not have focus. Such text tends to be rendered somewhat-darker-grey on somewhat-lighter-grey.

This is an accessibility problem.

I understand that it is possible to switch to the HighContrast theme (may need to do that when I get even older), write a completely new theme from scratch or maintain a fork of an existing theme that has all the backdrop definitions modified. These are not very good options though.

I was even considering fixing GTK+ run-time using LD_PRELOAD but neither do_net_wm_state_changes() (in gdk) nor gtk_window_state_event()/ensure_state_flag_backdrop() (in gtk) seem to have a reasonably patchable execution path. Run-time patching of the CSS selector to somehow avoid definitions involving "backdrop" is even less feasible. And of course, such patching has always a high risk of breaking things.

So, please add a way to disable backdrop and thus make text in unfocused windows readable again.

If one does not have multiple windows on screen then backdrop is pointless. When one does, presumably, it is because he needs to see their content simultaneously.