There seems to be a problem with the last update of xserver-xorg-core-hwe-18.04 or xserver-xorg-legacy-hwe-18.04 with Radeon graphics cards, which break the rendering of boxes and tooltips.
They just uploaded a fix in launchpad. Just update the packages to their newer versions
xserver-xorg-video-amdgpu-hwe-18.04:amd64 from 19.0.1-1~18.04.1 to
There's a problem with the xserver-xorg-core-hwe-18.04 update of Jan 16 and radeon graphics cards.
There are newer versions of the package to download. More explained version of this answer in this post.
Or download the newer packages in here.
UPDATE I might have an actual answer / workaround: booting with kernel 5.0 (instead of 5.3) makes these issues disappear. During boot, select "Advanced options for elementary", then "elementary, with Linux 5.0.0-37-generic" (instead of the default 5.3.0-26-generic option). With this older kernel, the screen update issues do not occur for me.
This would be expected behaviour, the & whilst making it a background task does still keep it as a child process of the terminal so when the terminal is closed so does plank. If you wish to keep it alive this question may be helpful: https://unix.stackexchange.com/questions/4004/how-can-i-run-a-command-which-will-survive-terminal-close#4006.
You can start to create your own plank theme.
If you just want a dark theme to use right away, look at https://github.com/ChildishGiant/elementary-dark-plank. It fits very well with the default dark theme on eOS.