Are you using the latest tumbleweed snapshot?
Your are still using the qt5ct. You can remove qt5ct in yast and install qt6ct.
Use custom instead of default and set it to darker.
this seems to be broken in kdenlive. I run Breeze Twilight (breeze dark for plasmothingies, breeze for widgets) on Plasma 6. The only way to have proper icons in kdenlive is to set global icon theme to breeze. It will then correctly flip to light icons on dark background for kdenlive the same as it does for the dark plasmoids. The “force breeze icon theme” needs to be off for that because that option will switch to dark icons on dark background. But since I do not like the boring breeze icons I am currently stuck with either missing icons for non-breeze icon theme or the dark on dark with kdenlive setting the icon theme
I can confirm this bug.
Something definitely wrong with the icons sets.
In “native” KDE suite applications the icons are looks good, for example in Dolphin, but in Krusader with dark theme, the dark icons are displayed, so they are barely visible. Similar to the KDevelop and Kdenlive screenshots above.