Yes, you can customize the appearance of the particles by modifying the `Particle` class. To change the color, size, or shape, you can use Kivy's canvas instructions within the `Particle` class. Here's an example of how to achieve this:

1. Import the necessary classes from `kivy.graphics`:
```python
from kivy.graphics import Ellipse, Rectangle, Line
```
Modify the `Particle` class to include a `canvas` instruction. You can set the particle's color, size, and shape within the `with self.canvas`: block:
```python
class Particle(Widget):
    velocity_x = 0
    velocity_y = 0

    def __init__(self, **kwargs):
        super(Particle, self).__init__(**kwargs)
        self.size = (random.randint(10, 40), random.randint(10, 40))  # Set the size of the particle
        with self.canvas:
            Color(random.random(), random.random(), random.random(), 1)  # Set the color of the particle
            # Choose one of the shapes below:
            Ellipse(pos=self.pos, size=self.size)  # Circular shape
            # Rectangle(pos=self.pos, size=self.size)  # Rectangular shape
            # Line(points=[self.x, self.y, self.x + self.width, self.y + self.height], width=2)  # Line shape

    def update(self):
        self.pos = (self.pos[0] + self.velocity_x, self.pos[1] + self.velocity_y)
```

In the example above, I've set the particle's size randomly within a specified range, assigned a random color, and provided three different shapes (Ellipse, Rectangle, and Line) that you can choose from. You can uncomment the desired shape and comment out the others.

Now, when you run the application, the particles should have customized appearances based on the modifications you made to the `Particle` class.
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