Oh nice! Switzerland here so we are neighbours I guess
...
Actually mine doesn't look exactly the same and I noticed something on the Picture here.
It gets stranger and stranger and I soon need sunglasses to watch my screen 
Yeah correct. Within the ubuntu install even when you go advanced custom install (partitioning yourself) ubuntu will care about the boot loader. You just have to choose the drive you want it to be installed and I chose the USB stick. I though if everything goes on the usb stick, I can't mess it up... I though wrong
Actually installing Linux on the external drive is not that simple as it seems because of this bootloader issue as you are experiencing at the moment.
The best way to test out the another distro is running a live session from a USB key. In this case, everything is loaded on the volatile memory, leaving no trace on your system.
So for some reason I don't have this line anymore
I didn't change anything there (just went down to GRUB_THEME with the arrow keys) but obviously I don't know if it was there before my Screen brightness Problem.
Yeah I know. Did that with many distros. But wanted to try and have a USB Stick as a small PC that I can carry around with me and store some files, download Programs and and and
If you want to have such portable Linux, you might want to check out Puppy Linux. With persistent partition, you can easily move your Linux environment from one machine to another.
For your reference, here's my grub file on that dual-boot laptop:
If you change this file, run 'update-grub' afterwards to update
/boot/grub/grub.cfg.
For full documentation of the options in this file, see:
info -f grub -n 'Simple configuration'
GRUB_DEFAULT="0"
GRUB_TIMEOUT_STYLE="hidden"
GRUB_TIMEOUT="10"
GRUB_DISTRIBUTOR="lsb_release -i -s 2> /dev/null || echo Debian"
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX="initrd=/casper/initrd.lz4"Uncomment to enable BadRAM filtering, modify to suit your needs
This works with Linux (no patch required) and with any kernel that obtains
the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"
Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL="console"
The resolution used on graphical terminal
note that you can use only modes which your graphic card supports via VBE
you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE="640x480"
Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID="true"
Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"
Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"
GRUB_THEME="/usr/share/grub/themes/zorin/theme.txt"
GRUB_DEFAULT is for me ="Zorin"
yours is GRUB_DEFAULT="0"
and the last of these GRUB line is just
GRUB_CMDLINE_LINUX=""
yours is GRUB_CMDLINE_LINUX="initrd=/casper/initrd.lz4"
everything else is the same except I have also these lines after GRUB_THEME
GRUB_SAVEDEFAULT="false"
#GRUB_HIDDEN_TIMEOUT="0"
export GRUB_COLOR_NORMAL="light-gray/black"
export GRUB_COLOR_HIGHLIGHT="magenta/black"
Any Idea on the checkmark for the customizer that is unchecked on Zorin Theme (but also for you)?
You do not have to pay any attention to the hashed (#) lines.
I think you can edit your grub exactly like mine, update grub then try reboot.
Even if it fails, now you got a mighty Boot Repair USB to fix it.
I also suggest to put hash (#) instead of delete the lines just in case.
Oh, and do not forget to update grub after edit.
I guess I will try tomorrow.
Yeah, i didn't see difference in the hashed lines but just in the ones all under each other that start with GRUB
Appreciate all the help. Hope it is not just "work" for you but also curiosity and some learning as well maybe.
Good night
Installing another operating system must affect Grub. Often, installing another OS gives grub priority to that OS, placing it first in the boot order list and its Theme being the one it refers to.
DO you still have Ubuntu Budgie installed? Because it may be looking to that build, instead of the grub on Zorin OS.
Nano is the terminal text editor. You can also use VIM.
sudo vi /etc/default/grub
Yeah, that is want I kinda knew. Everytime you install a new os it uses its grub but thanks for the explanation. I just didn't know that it will affect the bootloader if I tell the installer to write it on the stick.
Ubuntu Budgie is still installed on the stick, yes. But I didn't use it anymore after the initial install and first boot. I am a little afraid that when I plug in the stick and start from it, it will mess with the two OS on my pc again.
So without the stick it shouldn't affect it but it does.
I just don't see the logic behind that at the moment.
Did the installer think the stick is part of my pc ssd and overwrote the Zorin Grub although I chose a separat (the usb stick) drive 
That would be odd. Can you post a screenshot of the line in your Grub file showing the grub theme?
That aside... this is a BIG part of why I avoid Grub Customizer like plague... It also does these things and figuring out what it did and undoing it is like pulling teeth. I would recommend looking for the Grub Customizer configuration files and purging those, as well - that may be the solution, too.
Oops.
It was me who recommended it 
I wanted to change this entry of the available OSs
from Windows 10 -> Windows 11 and it worked like a charm for me.
I should think twice before I recommend it to others.
Where would I find these Grub Customizer configuration files (only if you have a guess or know it, don't waste time an look it up)
Yeah, it was your fault - all because of you. I wouldn't have tried to install Ubuntu Budgie on a Stick without you
Don't worry mate. All good. I appreciate all the help from you and Aravisian wheter it worked or not!
That is how everyone learns. By trying something new and maybe make mistakes. And not everything is good or bad for everyone.
Thanks, the screen shot helped a lot.
Your Grub Theme is commented out with a hashtag. Remove that hashtag (uncomment it).
Remove all four lines at the bottom. They can all go.
GRUB_SAVEDEFAULT < remove that whole line and the three lines below it, too.
Then ctrl+x to exit, the y key to say yes to save. Then enter key to save- terminal will revert to normal. Run
sudo update-grub
Then reboot and test...
Nothing changed after restart
Did I understand that correct? The Hashtag is for notes. So all lines with Hashtags don't affect anything?
Yes, the hashtag comments a line out, telling the system to ignore it.
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=10
Did you run sudo update-grub?
yes I did. But I am not sure if the outcome was what I needed.
Here are the lines I got:
sudo update-grub
Quelldatei `/etc/default/grub'
/usr/sbin/grub-mkconfig: 37: /etc/default/grub: Syntax error: EOF in backquote substitution
Edit: "Quelldatei" means something like "Source file"