Oh, I thought you meant one of those file tree sidebars was white.

Does the file picker look somewhat like this?

Image

Then it’s using the KDE file picker. I believe, it should be possible to make it use the GTK file picker, by configuring the “desktop portal” correctly.
Here is a guide for doing the reverse of what you need (GTK application in KDE Plasma): wiki.archlinux.org/…/Uniform_look_for_Qt_and_GTK_…
Maybe you can do the steps the other way around or it helps you find a better guide…

And no problem. 🙂