[RPN] Add a line on top of the input field

[yannis300307]

Is your feature request related to a problem? Please describe.
Compared to the calculation app, the input field in the RPN app doesn't have a border line.

Describe the solution you'd like
Add a border line.

Additional context
Calculation:

RPN :

This is my custom theme but you can see that the line is missing. It's less visible on the default themes.