can u please provide more info about the grub file u edited (path of the grub file)? if u edited the grub.cfg in the boot > grub directory, then u can reset the grub.cfg file by running sudo update-grub in the terminal. running the above command can fix the issue.
I didn't changed the icon theme. It came like this.
This is the command and path of the grub file which I edited:
sudo nano /etc/default/grub
While we could try sudo dpkg --configure -a and sudo apt install --reinstall plymouth - as this is such a fresh install of Zorin OS; I would just wipe and reinstall Zorin. Clearly, the install had a corruption or error during the process. And as this is a fresh install, you are in no danger of losing personal data and files.
So I have to start the installation process over from scratch for a fresh install, correct? using a bootable USB?
If it came like this, it means system's essential components are missing. So, you should reinstall it.
I actually chose the option to delete Windows and then install Zorin OS. This is the second option, as opposed to dual booting. So, how do I do steps 2, 3, and 4 of this guide?
Okay, I'll let you know if something happens 
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Yes. I recommend redownloading the .iso. Steps 3 and 4 can be ignored since Windows is not on the drive.
For step 2, you can access the BIOS EFI Settings from the motherboard. Usually booting then tapping F2, F10, F12 depending on the manufacturer.
Redownloading and reburning the .iso file is the safest bet - since something went wrong with this last attempt.
Okay, I just downloaded the iso file again and will do the rest of the things tomorrow and will update 
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Secure boot is missing from the boot tab in the BIOS, it only shows network boot and external device boot. External device boot is enabled and network boot is disabled.
Try the Security Tab.
It's not in the security tab too. I checked the whole boot menu. But couldn't find the secure boot option
What is your year, make and model of computer?
Year is 2011 I think. Model is Sony Vaio VPCEG3AEN.
Well... I have run a variety of searches.
Short of it is - I can find very little, including from the Sony website.
You may have a Minimalist BIOS Setting Screen and must enable Advanced Options for BIOS settings, first.
Looking that up led me to this Quora which was unnerving:
Oh no, this sounds difficult, and I won't take the chance. But in any case, I really appreciate your assistance with this. I now worry that I won't be able to return to the windows 
You can reinstall windows and then I suggest you to look to q4os if you want a Linux distro: it's installable directly from windows, without an USB or modifing (correct?) the os currently installed
I will look into it. Thanks for the suggestion.