If the libreoffice-gtk library is loaded, the menu highlight colours are not correctly loaded from the GTK theme. The highlight colour itself is correct, but the foreground colour used by native GTK applications is ignored. The following patch fixes this and uses the correct text highlight colour:

--- vcl/unx/gtk/gdi/salnativewidgets-gtk.cxx
+++ vcl/unx/gtk/gdi/salnativewidgets-gtk.cxx
@@ -3870,9 +3875,7 @@ void GtkSalGraphics::updateSettings( AllSettings& rSettings )
     }
 
     aHighlightColor = getColor( pMenuItemStyle->bg[ GTK_STATE_SELECTED ] );
-    aHighlightTextColor = getColor( pMenuTextStyle->fg[ GTK_STATE_PRELIGHT ] );
-    if( aHighlightColor == aHighlightTextColor )
-        aHighlightTextColor = (aHighlightColor.GetLuminance() < 128) ? Color( COL_WHITE ) : Color( COL_BLACK );
+    aHighlightTextColor = getColor( pMenuItemStyle->fg[ GTK_STATE_SELECTED ] );
     aStyleSet.SetMenuHighlightColor( aHighlightColor );
     aStyleSet.SetMenuHighlightTextColor( aHighlightTextColor );
 
However, this reveals another bug, which is that the highlight colour is also used for the menu header when active, which should not happen in a native GTK application. This is fixed by the following patch:

--- vcl/source/window/menu.cxx
+++ vcl/source/window/menu.cxx
@@ -2642,6 +2642,8 @@ void Menu::ImplPaint( Window* pWin, sal_uInt16 nBorder, long nStartY, MenuItemDa
             {
                 if ( bIsMenuBar && bRollover )
                     pWin->SetTextColor( rSettings.GetMenuBarRolloverTextColor() );
+                else if ( bIsMenuBar )
+                    pWin->SetTextColor( rSettings.GetMenuBarTextColor() );
                 else if ( bHighlighted )
                     pWin->SetTextColor( rSettings.GetMenuHighlightTextColor() );
             }