I'm not completely blocking this, but I have two concerns with this patch:

1. wallpaper is somewhat different to the other configs, I'm not super convinced we want to reset it without prompts

2. the resetting has potential to mess with 3rd parties setting the wallpaper.

From a customer project we have:
usr/share/plasma/look-and-feel/net.bluesystems.[redacted]/contents/plasmoidsetupscripts/org.kde.plasma.folder.js:applet.writeConfig("Image", "file:///usr/share/wallpapers/[redated]os.jpg")

Currently:
if you click on the lnf with reset layout you go to the lnf set one. That will result in it switching to the distro set one if available otherwise fall back to lnf
if you click on the lnf without reset layout you don't change the wallpaper at all

After this patch:
if you click on the lnf with reset layout everything is as before. That's all good.

if you click on the lnf without reset layout we will:

• if defaults exist and wallpaper is set, change it
• if defaults exist and wallpaper is not set, do nothing
• if defaults does not exist, load the wallpaper from the superclass lnf

Which might seem a bit random.

I have a feeling we'll get user complaints and bug reports if we unconditionally reset the wallpaper when applying a global theme. Before we do this, I think we need to add a mechanism to allow the user to choose which parts get applied, per the discussion in T11746: Make it possible to see what a Global Theme will change and only apply certain parts of it.