I'm not sure I understand the issue? You can make as many notebooks as you like.
Just press the + button and it makes a new one. You can then drag and drop it where you want.
Correct but my point is that the change is purely cosmetic, there is no change to the actual function of Joplin, all it is is an increased font size + an icon.
Unless we have .css support added to the mobile app somehow then it simply isn't possible. There have been discussions about mobile app theming + customisation but it isn't here just yet.
This is correct but, as @Daeraxa pointed out, this only applies to the cosmetic difference between how Joplin currently works and the picture in your original post.
The section header in the sidebar named "NOTEBOOKS" (note the plural) is not a notebook. It is the root for all the subsequent notebooks underneath it. This is just like the section header "TAGS" is not a tag; it is the tag root with all the actual tags listed underneath it.
So Joplin already has the ability to have multiple notebooks. You can also have sub-notebooks (which would equate to "folders" in your picture). So the actual functionality you have asked for is basically how Joplin already works and is available on all platforms that Joplin is built for.
Interestingly I was trying to have a go at putting in "tree lines" like that by modifiying the list item (the below was just a test to see if I could get a line to appear).
.list-item {
border-left: 1px solid white !important;
}
Ultimately I got bored and decided I didn't really need it, it wasn't very aesthetically pleasing to me.
As for the +/-, that is already a caret that can be changed to whatever you want e.g.