If you wanted a forest theme, you could do two, or three trees, depending on the shape of the wall - A half trunk on either edge and one in the middle, with intertwining branches coming off it.

You could do a "cityscape" theme, with some buildings ending at one shelf and others continuing up behind it, and the bottom shelf being the "top" of a fence - we know cats adore walking on fence tops.

You could go whimsical, with a sea-theme, and painted "kelp" supporting the shelves, and fishes behind

or abstract and just Mondrian-like blocks of color behind the shelves, and intersecting them ...

or Asian - bamboo forest with a tiger lurking between the canes

or literary - paint a bookcase with some books with cat-themed titles on the spots where the shelves don't reach ...

Need more?