In case that "obfusticated themes programmers bullshit" is too difficult to figure out for the OP, one could easily use a search engine.
Among the first results are some video howtos, even on how to change the wallpaper.
https://userbase.kde.org/Plasma/HowTo/4.5#DesktopTo use a custom one, simply click on open and choose your file.
Not sure how that's obfuscated or bullshit, and not sure how to make that easier. In any case, no need to keep this thread open.