I have the same issue on a 3ds I got from a relative, it might be damage from being in storage for so long. If anyone knows anyway to fix it short of replacing the screen, I'd love to know!
{
"L_BTN": "372d2d",
"L_JC": "ffafaf",
"R_BTN": "372d2d",
"R_JC": "ffafaf",
"name": "Pastel Pink (Princess Peach)"
}
In case you wanted the new joycons colour codes, I just freshly dumped mine