From the four colors:
- highlight-background-focused (orange)
 - highlighted-text-focused (white)
 - highlight-background-no_focus (light gray)
 - highlighted-text-no_focus (black)
 
The auto-comp box currently uses highlight-background-
no_focus (light gray) and highlighted-text-focused (white), which is why it is completely unreadable.  If we make 
no attempt to give the box the "focused" color set, it becomes readable:
Index: src/sdk/wxscintilla/src/PlatWX.cpp
===================================================================
--- src/sdk/wxscintilla/src/PlatWX.cpp	(revision 8782)
+++ src/sdk/wxscintilla/src/PlatWX.cpp	(working copy)
@@ -951,7 +951,7 @@
         // can't receive focus then we have to pull a fast one and temporarily
         // parent the listctrl on the STC window and then call SetFocus and
         // then reparent it back to the popup.
-        lv->SetFocus();
+        //lv->SetFocus();
         lv->Reparent(this);
 #ifdef __WXMSW__
         lv->Show();
This, of course, is not the desired appearance, but I guess it implies that highlight-background-* and highlighted-text-* are set differently, and (possibly) other themes work because mismatched sets of these are still readable in those themes.
I do not currently have wx29 on Linux, and have not tested it yet.