I swear it's not complete. It's made in 15 minutes. If you want, copy paste the code in console of any profile on GitHub.

game.js

let el = document.getElementsByClassName('js-calendar-graph-svg')[0].children[0]

let interval = null;
let refreshInterval = 200;

let directionVectors = {
  37: {x: -1, y: 0},
  38: {x: 0, y: -1},
  39: {x: 1, y: 0},
  40: {x: 0, y: 1}
}

let initialPosition = [20, 0]

let currentVector = {x: 1, y: 0}

document.addEventListener('keydown', function(e) {
  if(e.keyCode === 37 || e.keyCode === 38 || e.keyCode === 39 || e.keyCode === 40) {
    e.preventDefault();
    currentVector = directionVectors[e.keyCode];
    clearInterval(interval)
    interval = setInterval(move, refreshInterval)
  }
})

function move() {
    clearPreviousHead()
    initialPosition[0] += currentVector.x;
    initialPosition[1] += currentVector.y;
    setSnakeHeadPosition(initialPosition[0], initialPosition[1])
}

function clearPreviousHead() {
  el.children[initialPosition[0]].children[initialPosition[1]].style = ""
}

function setSnakeHeadPosition(x, y) {
  el.children[x].children[y].style = "fill: red";
  el.children[x].children[y].dataset["level"] = 0;
}