To fix these two problems enter:
sudo chmod 666 /usr/share/themes/io.elementary.stylesheet.*/plank/dock.theme
Note with the * we´ll also catch the stylesheets
banana, blueberry ... strawberry
The error:
Missing key 'UrgentHueShift' for group 'PlankDockTheme'
will be fixed as well since now plank can save the file and set the missing setting.
Well before I ran the "change file mode bits" I check with:
ls -al /usr/share/themes/io.elementary.stylesheet.blueberry/plank/dock.theme
the current settings:
-rw-r--r-- 1 root root 2600 Apr 25 20:31 /usr/share/themes/io.elementary.stylesheet.blueberry/plank/dock.theme
So the file is owned by root (with read write access right) but used by user (in the group others last 'r--' with the read-only right)
Well some other approach is to change the owner from root to tux. (Replace tux with your username!)
sudo chown tux:tux /usr/share/themes/io.elementary.stylesheet.*/plank/dock.theme
Maybe this create some problems if apt-get ( or under the hood it`s dpkg doing the job) likes to update or remove these files but fails because of missing access permissions. However I have not tested if that is the case.