I have changed the system icon theme using the gnome tweak tool. All icons except the desktop icons and dock icons have changed. Is it possible to change these icons also?

Did you restart Shell?

It's possible that the theme you chose did not include icons for the desired entries. AFAIK the only way would be copy the icons you want into the directory with those from the theme. It's not possible to have two active icon themes.

The icon theme is Faenza which has all the icons in question. Restarting the computer did not change anything. The dock and launcher icons are Deepin and the Nautilus icons are Faenza. I was able to move the Deepin icon theme folder in usr/share/icons to the desktop. I then created a new Deepin folder in usr/share/icons and pasted the contents from the Faenza icon theme into that folder. That changed most of the icons after a restart. I will have to find the launcher icon, the show desktop icon, and the files icon in the Deepin folder because they were blank on the dock (the launcher icon was actually a Ubuntu icon). Maybe I will try replacing just the icons I don't like and see if that works.