It's possible.
The background color comes from the tcp_recinput.png image.
Red arrow points to the 1 pixel vertical line which is not covered by the pink line. So when the image gets stretched to form the background for the input text, it's that 1 px wide area which gets stretched.
The record input text is defined in the rtconfig.txt file inside the theme package. Example from the Default v5 theme:
set tcp.recinput.color ?recarm [255 255 255] [210 210 210]
The RGB numbers inside the first brackets are for when the track is record armed, inside the second brackets in other states.