TL;DR

cd isolinux #or wherever your files for ISO might be
BASE_URL=https://raw.githubusercontent.com/nadrimajstor/syslinux-themes-ubuntu-mate/master
wget $BASE_URL/ubuntu-mate-syslinux-gfxboot.cfg -O gfxboot.cfg
wget $BASE_URL/ubuntu-mate-syslinux-splash.png -O splash.png
wget $BASE_URL/ubuntu-mate-syslinux-splash.pcx -O splash.pcx
wget $BASE_URL/ubuntu-mate-syslinux-blank.pcx -O blank.pcx
wget $BASE_URL/ubuntu-mate-syslinux-access.pcx -O access.pcx
gfxboot -a bootlogo --add-file gfxboot.cfg splash.png splash.pcx blank.pcx access.pcx
# Rebuild ISO

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Now its left just to add icing on the cake... Small patch to the gfxboot-theme-ubuntu package to have a correct order of color depths (we want to make a new init file for our bootlogo)

=== modified file 'common.inc'
--- old/common.inc	2011-03-28 18:46:25 +0000
+++ new/common.inc	2014-08-19 20:56:04 +0000
@@ -1355,9 +1355,9 @@
 
 /transp 0 def
 
-640 480 16 findmode setmode not {
-  640 480 32 findmode setmode not {
-    640 400 4 findmode setmode not {
+640 480 32 findmode setmode not {
+  640 480 24 findmode setmode not {
+    640 400 16 findmode setmode not {
       false .end
     } if
   } if

And we have a 🍰

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All the steps documented 'cos we gonna redo some for live-image-mate-desktop.

Destination paths are relative 'cos I'm standing in the isolinux directory.

cp /usr/lib/syslinux/isolinux-debug.bin isolinux.bin
#or
cp /usr/lib/syslinux/isolinux.bin isolinux.bin #for production

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cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/isolinux.cfg .

---

cp /usr/lib/syslinux/gfxboot.c32 .

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cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/menu.cfg .

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tar -xf /tmp/gfxboot-theme-ubuntu/install/bootlogo.tar.gz

• This should come from gfxboot-theme-ubuntu: /usr/share/gfxboot-theme-ubuntu/bootlogo.tar.gz but at the moment there is some bugchaseing and I built it from source.
• Note that cpio archive bootlogo contins only one file init. With the help of isolinux bootloader we have access to a filesystem, however for other usages, some files that reside in our isolinux directory might need to be added also inside the booglogo cpio archive.

---

So it works... Ok then lets check on dependancyes.

isolinux.cfg:

• default vesamenu.c32

cp /usr/lib/syslinux/vesamenu.c32 .

In menu.cfg we asked for:

• stdmenu.cfg

cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/stdmenu.cfg .

• txt.cfg

cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/txt.cfg .

• gtk.cfg

# Can't find it anywhere / don't have a clue reagarding its purpose

• adtxt.cfg

cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/adtxt.cfg .

• adgtk.cfg

# Can't find it anywhere / don't have a clue reagarding its purpose

• prompt.cfg

cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/prompt.cfg .

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stdmenu.cfg:

• splash.png

wget https://raw.githubusercontent.com/nadrimajstor/syslinux-themes-ubuntu-mate/master/ubuntu-mate-syslinux-splash.png -O splash.png

txt.cfg:

• /casper/vmlinuz do not go to isolinux directory - ignoring for now
• /cdrom/preseed/ubuntu.seed do not go to isolinux directory - ignoring for now
• /casper/initrd.lz do not go to isolinux directory - ignoring for now
• /install/mt86plus do not go to isolinux directory - ignoring for now
• localboot 0x80

cp /usr/lib/syslinux/chain.c32 .

prompt.cfg:

• f1.txt
• f2.txt
• f3.txt
• f4.txt
• f5.txt
• f6.txt
• f7.txt
• f8.txt
• f9.txt
• f10.txt

cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/f{1..10}.txt .

• exithelp.cfg

cp /usr/share/syslinux/themes/ubuntu-trusty/isolinux-live/exithelp.cfg .

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wget https://raw.githubusercontent.com/nadrimajstor/syslinux-themes-ubuntu-mate/master/ubuntu-mate-syslinux-splash.pcx -O splash.pcx
wget https://raw.githubusercontent.com/nadrimajstor/syslinux-themes-ubuntu-mate/master/ubuntu-mate-syslinux-gfxboot.cfg -O gfxboot.cfg
wget https://raw.githubusercontent.com/nadrimajstor/syslinux-themes-ubuntu-mate/master/ubuntu-mate-syslinux-blank.pcx -O blank.pcx
wget https://raw.githubusercontent.com/nadrimajstor/syslinux-themes-ubuntu-mate/master/ubuntu-mate-syslinux-access.pcx -O access.pcx