To install Appimages from Appimagehub via CLI use appimage-cli-tool. This allows you to e.g. search appimages with "app search xyz" and install any appimage via command "app install #id".
If you are a developer who is interested to easily build appimages from your code repo, read the guide here.
Plymouth is a bootsplash for Linux supporting animations using Direct Rendering Manager and KMS driver. It gets packed into the initrd. Besides eye-candy, Plymouth also handles user interaction during boot. Wikipedia
copy YOURTHEME folder to usr/share/plymouth/themes/YOURTHEME