iBooks dark theme background is grayish, not black

Lately I updated to El Capitan, and I realised the iBooks dark theme has been changed (or added, I can't remember if I used "dark theme" or "invert colors" option). Anyway, now my iBooks has a dark, but grayish background - it's not black anymore. I read in complete darkness at night, and now the screen actually glows which drives me crazy...

Does anyone know a way to downgrade iBooks to previous version, or ideally customize background color? I'm not talking about switching theme to "dark", but to actually change the 0x000000.