 
    There will be missing icons when installing the KDENLIVE package only on Ubuntu. Ubuntu’s KDENLIVE package assumes that you will install other KDE-related packages, including “breeze.” “Settings|Force Breeze Icon Theme” is turned on, by default.

Therefore, you should turn “Force Breeze Icon Theme” off and restart KDENLIVE.

Voila, the icons will re-appear!


However, since missing “Breeze” is the problem, why not install it?
sudo apt install breeze breeze-cursor-theme kde-style-breeze kwin-style-breeze -y
Now, you can choose “Color Theme” options like “Breeze,” “Breeze Dark,” “Breeze High Contrast,” and “Breeze Light.”

Just in case, you can “RESET KDENLIVE” by deleting the “kdenliverc” file.
$ ls -al $HOME/.config/kdenliverc 
-rw------- 1 cbkadal cbkadal 3706 May 30 12:41 /home/cbkadal/.config/kdenliverc
$ rm -f  $HOME/.config/kdenliverc 
$ 
This is the way!