Comment # 1 on bug 1187477 from Shawn Dunn

In Leap 15.4/LXQt 1.1.0 I can't reproduce the behavior you are describing, the
themes all apply just fine on my end.  The wallpapers also change, provided you
tick the "Override User-Defined Wallpaper" checkbox in LXQt Appearance
Configuration.

Is this still an issue for you?