Getting right dropdown icon look for evolution in KDE

KDE - I’m still having troubles with understanding and setting global theme, application style, plasma style, icons, etc.

I found the Oxygen Icon style mostly tolerable. However in Evolution, the dropdown boxes on the message preview show as blanks. I found by selecting Breeze or Adwaita icon, it shows up. Here are the examples in order of Oxygen, Breeze, Adwaita:
15.5-evolution-oxygen

15.5-evolution-breeze

15.5-evolution-adwaita

I most prefer the Adwaita dropdown arrow. But all the rest of the icons for other things look really ugly. Is there a way for copying Adwaita dropdown to the oxygen set?
I believe I found Adwaita icons in /usr/share/icons/Adwaita/scalable/ui/pan-down-symbolic.svg and pan-end-symbolic.svg
But I don’t know what to do with them since I can find no comparable location for oxygen. I suppose since it’s missing. But neither can I find the relevant icons for Breeze.

Evolution is GTK based, Then you add themse and icon sets look for some GTK styled ones for plasma. There should be some that will give you the esthetic you are looking for.

Okay, I tried some. Some of the time on gtk se98-wm-theme-main, it would have the little down arrow symbol. But when I changed a non-gtk item, it disappeared and I never could get things changed back. I didn’t like the other things of it anyway.

I was hoping that I could change just the one item. Something like ~/.themes/se98-wm-theme-main/gtk-3.0/gtk-arrows.css. I see something in there with -gtk-icon-source: -gtk-icontheme(“pan-down-symbolic”); but since that’s already set, not sure why it’s showing blank.

If someone could tell me which themes have the down arrow for evolution, that’d be nice. I’ve tried several and logged in and out multiple times with the changes to see if it does anything useful, but the process seems to be a waste of time on randomness.

I found the following on the web

Evolution doesn’t have its own themes. Selecting a dark GTK+ 3.0 theme should cover most of the UI, but mail and composer views may not follow as they are web views.

I don’t know what that means, web views, and how that enters into the arrow part.