it seems previous versions of kdenlive had an option to enforce the Breeze icon theme instead of the system’s Qt icon theme, but apparently it’s been removed?
I’d like to see this added back, as my icon theme overrides a bunch of the tool icons with its generic kdenlive app icon, and manually changing them all back to the defaults with the Settings > Configure toolbars... menu would take forever (not to mention the icon picker doesn’t even work, you have to manually select the icon file yourself). I also couldn’t find a way to generically set the icon theme for a specific Qt application as a workaround.
tried this by adding Kdenlive to the “Kvantum (default)” theme in “Application Themes” and running it with QT_STYLE_OVERRIDE=kvantum and it didn’t work; it only changed the Qt theme, not the icons.
My bad. I saw “theme” and completely omitted the “icon” part from my mind.
To my knowledge, this is an app-level thing only. Some Qt apps, like qBittorrent, can set their own icons - GTK apps too (GIMP and Inkscape). But for the most part, if the app’s config files don’t specify an icon theme, like dolphinrc, then you can’t change it.
I know this doesn’t help you right away, but it points towards the icon set you are using that seems to have some icons missing or not categorized properly. Best option is to contact the maintainer of your icon set.