After a reinstall of the whole System Add/Remove Software, Manjaro Hello and some other programs look like Gnome applications on my KDE desktop. I already repeated the installation and tried to reinstall several packages after that, which did not solve the issue. Although this is purely a cosmetic thing, I would be glad, if somebody can help.
Well, those are GTK-based applications, and GTK is the widget toolkit that GNOME and GNOME applications are written with, while KDE Plasma is written with the Qt toolkit.
The best you can do, if you really want to give them an as close as possible feel as Qt, is use the Breeze and/or Breath themes. Anything else will always cause Qt and GTK to deviate sttrongly in look & feel.