(Linux Mint Mate, 18.mumble---based upon Ubuntu 16.04, on intel x64 machine)
Ok, having tested this side-by-side with 2.4, what happens is this:
1. WORKING: When I [re] install 2.4, while on a Darcula theme if I import my editor color settings (and nothing else), all is fine. I keep the Dark theme for everything but the actual editing window, which is customized to my parchment background with black font.
2. NOT WORKING ANY LONGER(?): When I install 3.1, I import from 2.4, and get a full Darcula Theme with the editing background still black. When I import just the editor colors, I see a FLASH to a parchment editing background which is then immediately squashed by the Darcula colors.
It's as if in 3.1, you now reset the dark theme after import when you didn't do that in version 2. Before, in 2.4, once the editor colors were set, they could be reimported on any reinstall of 2.4 and my background was still parchment.
BTW, same goes for the syntactic coloring, but one thing at a time.