Varying image based on content length

I'm looking to script a field that places different images based on how much text is in another field (in this case, "Theme"). The total length can be between 0 and 4 characters; if there are 0 characters then the field is invisible, so I just repeated image 1 for both 0 and 1 character length.

I've read the documentation but I'm not sure what to use to establish this. content_length seems to be the incorrect term, as does number_of_items. I'll put my code below. Help would be greatly appreciated!!

field:
top: 0
left: 0
height: 402
width: 298
z index: 3
visible: { if card.Theme != "" then true else false }
render style: image
choice images:
default: { if card.Theme.number_of_items == 0 then symbol-mask.png else if card.Theme.number_of_items == 1 then symbol-mask.png else if card.Theme.number_of_items == 2 then symbol-mask-2.png else if card.Theme.number_of_items == 3 then symbol-mask-3.png else if card.Theme.number_of_items == 4 then symbol-mask-4.png }

The intended result would be something like
theme: A (results in image 1)
theme: AB (results in image 2)
theme: ABC (results in image 3)
theme: B (results in image 1)

theme: BC (results in image 2)

etc.