So, the problem persists but I think I found the cause. Within the compressed files (and this is for the Windows and PC builds) there appears to be two "textbox.png" files for some reason. When extracting the .zip, the program overwrote one with another, leading to only the white remaining in the decompressed file.
This might be something happening with my machine or one of Ren'py's silly little oopsies, but I suggest you check out how it looks on your end.
Be as it may, the game is working fine for me now. Cheers.