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.
How to install a grub theme:
Extract the theme to /boot/grub/themes/
Edit /etc/default/grub file. Find the line starting with "#GRUB_THEME" and change it to "GRUB_THEME=/boot/grub/themes/[THEME_FOLDER]/[theme.txt]" (replace [THEME_FOLDER] ; [theme.txt] is probably OK ; remove "" brackets)