While wrestling with making a new theme, I suddenly had two different coloured right click menus.

I'm not sure how I got there. Nor what I should look for to change it back.
Can anybody help my in the right direction?

QtCurve or Kvantum? It looks related to menu transparency.

That did it! Thanks!

The problem got solved by setting the "menu transparency slider" to "opaque" in Application Style -> Widget Style -> Configure... -> Transparency.
To keep menu transparency I adjusted it in Desktop Behaviour -> Desktop Effects -> Translucency.

The over crowded settings are getting better, but it seems plain weird with multiple settings for the same effect, different places, that can conflict with each other.
I would love to know the technical reasons behind it. Different tool kits or ... ?

Nevertheless; solved!