I'm running up-to-date KDE (i.e., Plasma 5.2.3.4 on X11) on Arch Linux. When Dolphin is in Split View mode, the background of inactive panel is indistinguishable from the active regardless of the themes and theme tweaks (per everything I've tried).

This is a NEW issue. See http://www.linux-databook.info/downloads/Dolphin_guide.pdf, page 9, figure 3 for design intent. Also, see this capture of Dolphin running in KDE Neon 5.20, https://i.imgur.com/Vi0ndlk.png. So, it was working last year. And even on a Chromebook, the most up-to-date Dolphin has distinguishable panels (e.g., https://imgur.com/a/AdsQXKo), but of course it runs w/o Plasma on a Chromebook; that indicates it is a DE defect, not Dolphin's (I think).

Oddly, the only control for the color of the inactive panel in KDE Neon 5.20 seems to be "Common Colors" / "Window Background", and the effect seems to be making the color of the inactive panel fall between the active panel and the window background. Quite the strange control.

I know there are other subtle differences between the two panels, but losing the slap-in-the-face indicator (i.e., the panel background difference) makes using Dolphin more error prone. So, the immediate question is there a new way to make the Dolphin's Active / Inactive panels have different colors? Or is this a Plasma defect? Or what?

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P.S. Given no help isolating the issue or a work around, I appended a comment to https://bugs.kde.org/show_bug.cgi?id=150718 (split view - difficult to understand which is the active panel) although unsure it is a Dolphin issue.