So we changed what you did in start-up manager back to when it was a working system and we updated grub to apply it and we removed and reinstalled plymouth
because you attempted to install a theme that was not in Synaptics. You replaced grub2 into the mbr. The first 2 were the last 2 things you did before it stopped booting.
Well we better look at grub. This will give you the whole grub config in a file on your desktop to post here. Lots of users can read.
Download this to desktop and then run the code below takes about 30 seconds. Will tell everything about grub2 in your system.
Must be Downloaded or on Desktop and easier to read when you post (is long) put code tags before first and after last words or highlight and hit # sign in upper right.
SourceForge.net: Boot Info Script - Project Web Hosting - Open Source Software
And you have started in recovery-mode in low graphics mode?
sudo bash ~/Desktop/boot_info_script*.sh
And this below will give all your cards including graphics to other users to help get
you running. Dells have proprietary drivers and lots of Dell users out there.
Post these boot script and cards. There are a lot of excellent users on these forums and these will give them the info needed.
sudo lspci -v | less