Cat have 9 lives, remember? 
Right, as you know I use my own grub. And I've put in your prettygrub in it and it works. And I've shown the screen shots somewhere.
As for horizontal-grub, it is required that you put the file in /usr/local/bin
and do the command 'horizontal-grub' after making it executable. That I understand. I also understand that it will modify grub.cfg while using the 'prettygrub' theme. Correct so far?
Now my question to you is
- Does that modify 'prettygrub' ?
If it does, what part of theme.txt does it modify?
- I understand it modifies grub.cfg, so I ask if you can provide your grub.cfg so I can modify mine (with adjustments to my case).
As explained, I use my own grub and I can easily do this as I had with your (non-horizontal-grub) prettygrub. Of course I could try with doing the whole thing at the OS (instead at my own grub) but since I have this pretty grub at my own grub, it is easier (and easily convertible back to any theme
) to do it at my grub.
Hope this is now clear.
Cheers.
ps: as you also already know, it is not critical to me that I try this out
but to test out and give support to my buddy's 'venture'.
ps: my feedback as follows and hope it helps and not to criticize 
o The 'box' at the horizontal-grub menu selection (the dark olive green worn-out military ammunition box) is ugly (to me). At pretty theme, we can change it to using something more 'pretty'.
o And it is not centred but that depends on the resolution and monitor size used. Suggest we fix it at 1024x768.
o The heading at Linux Mint shows 'Manjaro' and the heading at Manjaro shows 'Linux Mint'.
(The red line at your screen shots.)