I'm running near-stock Kubuntu 16.04. I didn't fiddle with designs and/or themes (yet). I simply changed the system theme from "Breeze" to "Breeze Dark". 
After a reboot almost everything had accepted the new theme (title bar, task bar, etc). Except for dolphin. 
I googled around a little bit, but found no real clue.

Here are two screenshots (icons view is barely readable but details view (which I like most) is completely unusable)
icons view:    https://share.gingerlabs.de/screen_2016-07-03_01-01-25_qjFCvRqp.png
details view: https://share.gingerlabs.de/screen_2016-07-03_01-13-57_W9qqkvi5.png

It seems like only the viewport background changed and nothing else. The font and menus stay how they were before. (Font black (on black ground) and menus normal (as in black text on white ground instead of the other way around).

I tried everything that my limited knowledge allowed, but even downloading themes and changing back and forth (with reboots) didn't accomplish anything, as did playing around with the "-style" parameter

Reproducible: Always

Steps to Reproduce:
1. Change system theme to "Breeze Dark"
2. Restart
3. Open Dolphin

Actual Results:  
Dark theme only applied to the viewport background, fonts stayed black, menus stayed light-colored

Expected Results:  
Dark theme should apply to whole window, black backgrounds, white fonts

Edit: Just noticed that the system settings had the same error as well... weird
And yes, the color theme (System Settings > Colors > Scheme) is set to "Breeze Dark".