WPS Office is a Qt-based application; while GNOME is a GTK-based desktop environment. GTK theming does not work well with Qt applications. You need to configure the latter separately with qt5ct or a similar tool, and if you set the Qt theme to kvantum ─ which is a theme engine ─ then you can load kvantum themes into the Kvantum Manager for further tweaks to the look & feel of Qt-based applications.
I am facing the same issue too. It persists with all QT applications I am currently using. I thought I messed up some configs of mine and tried reinstalling applications, theme engines and the themes themselves.
I guess its a bug from the recent update. Was fine earlier (yesterday before an upgrade). Hoping for a fix soon.
Here is a glitched out version of Kvantum itself while using the Matchama-Dark Theme xD
i (dot) imgur (dot) com/zN0Ky1Z.png
Looks like it might be the same issue as some of the other ‘special cases’ that are on the opaque line of the .kvconfig file. But I havent been able to investigate just yet.