We might need to be more careful about toning down "content". :backdrop should flatten and tone down the chrome, but actual video/ images and text should retain contrast.

I will appreciate that for my day work. Thanks for considering.

I see what you mean, I think the ideal solution would be to tone down chrome only leaving content alone, problem is that we don't know what content is by the theme, probably a .content style applied to actuall content would allow us to properly solve this issue.

I think I have a solution, basically I resurrected text color, which opposed to fg color doesn't get dimmed much. So the fix I'm pushing uses (yet another) color for text entries and views in general, I used black (white in the dark variant) to increase contrast some more and it gets just sligtly dimmed in backdrop, so the end result is a backdrop window with, say, button text, labels and various controls text quite a bit dimmed to have a nice differentiation from the focused window while the content remains pretty contrasted so easy to read.
Unfortunatelly I had to add back @theme_text_color and it's backdrop variant (@theme_unfocused_text_color) to the API level exported color, but I guess it's unavoidable.
Carlos tell me what you think.

Fixed on https://git.gnome.org/browse/gtk+/commit/?id=062f450709c4b365fc76d82bbed7fec6048ea134