At Settings -> Configure Theme -> Window List -> Tree Window List -> Background -> Selected. This config take no effect. Issue cofirmed in version 4.2.0, specially the portable one from https://portableapps.com/apps/internet/kvirc_portable which I use.
Other bug is a strange entry in taskbar called kvirc4_ipc_sentinel, that is impossible to close.
See if it's a bug in the base app itself. If so, report it to the publisher. We're only responsible for the portable bits.
Sometimes, the impossible can become possible, if you're awesome!