Grub themes, how do you make them? Where do you get started?

So I want to make a grub theme with a menu similar to this one and it would be glossy and purple. That on a different background.

So how do I do that? Where can I get started? Is there any documentation?

Not sure if this is related to the forum.

if you just want to change the background image of grub thats not to hard.
just go to /boot/grub2/ themes/ , in themes you should find a folder of your distrobutions name ,like suse
or something , in that folder you’ll find all the different parts that make the screen . one of which will be a
( png ) image of what ever grub is currently showing . also you should find a document that says ( theme.txt )
to change the display image ,simply put the image you want to display in the folder . the image size should be
around 1024 x 700 pixels or so . Next open the theme.txt with a text editor like kate or Vi , you will need root
privilages to save the edited text . when the theme.txt is open in your editor ,look for a line that says ,
desktop-image:PHOTO NAME.png ( PHOTO NAME ) will be the name of the image that is currently
being displayed . change that line to read your image file name , like little pink horse.png
ie. " desktop-image: “little-pink-horse.png”
save the file , close the editor and reboot. your new grub image should be displayed .
one more thing , make a backup of theme.txt before you change it . just copy it to a safe folder in your
home directory so that you can put it back if you want .
Hope this helps

The background is pretty easy actually, but I wanted to know about the menu and selection like on this one it’s different than the regular one if you know what I mean. After some research I found that they’re called styled boxes. I think I probably found a way to do it like you must cut your box into parts and give them specific names, didn’t try it yet.