After installing Debian 10.4 with the KDE Plasma desktop on an older machine (32-bit, ASUS motherboard, P4 microprocessor, nVidia GeForce FX5200 graphics card), I can call up System Settings, but the dialog box is solid black. I can access Window Manager Settings, which brings up Configure - System Settings Module, but I am unable to change the color scheme. Also, the borders render as black. This has now happened twice for on two identical machines. Perhaps I cannot use KDE Plasma with this graphics card.

(I have been using KDE Plasma successfully for years on openSuSE - now running 5.12.8 on multiple platforms.)

Suggestions for command line entries would be appreciated. And of course I will provide any additional information needed.

Hello.
Same problem here, FX5500 using nouveau. Got black background and other issues that makes KDE unusable. I can't even see anything in the settings. Did you manage to solve the issue?

No, I don't recall resolving the issue. The platforms were older 32-bit machines and I abandoned the installation effort. (In addition to the ever-diminishing number of 32-bit applications, most of the Linux distros have discontinued 32-bit support and it was too frustrating to install the few that were still available in May 2020. Going forward, I will opt for 64-bit platforms.)

From my notes, I did find the following post which might help: https://forum.kde.org/viewtopic.php?f=17&t=166224