I mean #channel windows, private windows and probably all windows that can be created in the client.
Dark style is the ability to change the background by setting the image or changing the color in - ToolBar, SwitchBar, NickList, EditBox, Channel, Private ...
If all these parts of the client are made in dark colors, then the frames of the window in which the text of the chat is displayed will become visible.
Especially if these windows are slightly moved away from the control panel on all sides.