EBNF special borders are not very visually distinct from literals:
https://www.plantuml.com/plantuml/uml/SoWkIImgIKtAI-DAoyzNi5LGoiaiIIrAp55Ig56mBov8JSvCpB47iejASrAjkHnIyr90YW40
I think if we use the dashed version of the UStroke for one but not both rectangles... new UStroke(1.0,1.0,1.0)??, we would have something easy to understand at a glance... the numbers might need tweaked a bit... open to anything that makes the types of the components in the above diagram distinct from each other "from a distance".
https://github.com/plantuml/plantuml/blob/48ae51e8c3ee3cd5713834350f682b086db208d8/src/net/sourceforge/plantuml/ebnf/ETileBox.java#L160