irregular UI behavior

[ExecutorElassus]

I have two apps that are doing some odd things with their UI, and some general inconsistencies I'd like to resolve with my desktop. My WM is fvwm3, which appears to be using a mix of Qt and GTK for theming and widgets. Here's what I'm dealing with:

With net-p2p/transmission-4.0.6, set to use gtk and not qt6, when I spawn the window to select a download location, clicking the dropdown does not spawn a dropdown menu, but instead spawns a new, separate file-selection window. This is new behavior, ever since transmission updated to using GTK4.

With app-office/calibre-7.19, using Qt6, none of the scrollbars -- nor any of the UI buttons on the main calibre window nor in the reader application -- respond to clicks with the primary mouse button. The button shows the button-clicked animation, but there's no action performed. Scrolling with the wheel works, and clicking menu items from the right-click menu in the main calibre window also works. Clicking the "Help" button spawns the calibre manual webpage in a web browser, but no other buttons seem able to spawn their respective windows or menus.

Also, a general issue, my UI seems to use two different mouse iconsets, depending on the app. On this browser (Opera) it's one that is also used with the steam client, while every other app uses a different one. How do I isolate which theme-manager (GTK3? GTK4? Qt5? Qt6?) is setting the mouse, and how can I make it universal? More generally, how can I make sure every app on my system is using the exact same theme? Right now it seems to be a mix of GTK3, GTK4, and Qt5/6, and I'd rather it be completely consistent.

Cheers,

EE