In KDE, Dolphin does not show the XDG icons for the user directories in my home folder, e.g., Music, Downloads, Documents, etc. It shows the generic folder icon for all directories. How can I make Dolphin show the standard icons from the current icon theme?
Sadly, you cannot do this with Dolphin and Oxygen.
IIRC, Dolphin does not support showing special icons for these folders. It does support previews and tags however, but that's probably all you will get.
The KDE can use whatever picture (*.png *.xpm *.svg *.svgz) as a directory icon.
With the KDE file manager (Dolphin): Right click the icon > Properties > Left click the Icon (General tab) > Select Icon.
The setting are save to the .directory file.
KDE .directory file:
The .directory file is a KDE folder view properties file. It contains the folder viewing settings for a directory in KDE. It includes settings such as icon positions and view mode (icon, list, etc.). Automatically created when the view settings of a folder are modified. Hidden by default.