How to customise rEFInd theme on High Sierra?
I could be wrong, but isn't 

echo "include rEFInd-minimal/theme.conf" &gt;&gt; refind.conf 


suppose to be the following.

echo "include themes/rEFInd-minimal/theme.conf" &gt;&gt; refind.conf


You omitted the themes folder, but there are many references to a themes folder in the theme.conf file. 

I would suggest the following.


  From OSX, do the following
  
  Start by mounting your EFI partition.
  
  sudo mount -t msdos /dev/disk0s1 /Volumes
  
  Then cd to the rEFInd directory on that partition cd
  /Volumes/EFI/rEFInd
  
  mkdir themes
  
  cd themes
  
  Download the theme with Git
  
  git clone https://github.com/EvanPurkhiser/rEFInd-minimal
  
  Now add the theme info to refind.conf
  
  cd ..
  echo "include themes/rEFInd-minimal/theme.conf" >> refind.conf
  
  Reboot with sudo reboot