Default settings on my monitor made this hard to see too. It's because it is transparent. You can redefine the function that draws the screen by entering:

    function drawArena() {
    	for (var y = 0; y < CELLS_Y; y++) {
    		for (var x = 0; x < CELLS_X; x++) {
    			if (goal.x == x && goal.y == y) {
    				context.fillStyle = "rgb(0,127,255)";
    				if (arena[y][x] && !generations_until_beaten) {
    					winLevel();
    				}
    			} else if (arena[y][x]) {
    				context.fillStyle = "rgb(0,0,0)";
    			} else {
    				context.fillStyle = "rgb(255,255,255)";
    			}
    			context.fillRect(x * TILE_WIDTH, y * TILE_HEIGHT, TILE_WIDTH, TILE_HEIGHT);
    		}
    	}

    // Draw playable zone (if applicable)
    	if (playable.width && playable.height) {
    		context.fillStyle = "rgba(255,127,0, 0.3)";
    		context.fillRect(
    			playable.x * TILE_WIDTH,
    			playable.y * TILE_HEIGHT,
    			playable.width * TILE_WIDTH,
    			playable.height * TILE_HEIGHT
    		);
    	}
    }

in the javascript console (where 0.3 is the new transparency. It won't show up until you click the next level button.

I've increased the opacity in the game.

Awesome, thanks!