canvas如何做出黑色背景的青色煙火

<canvas></canvas><h1>201<span>8</span></h1>

html,body {    padding: 0px;    margin: 0px;    background: #222;    font-family: 'Karla', sans-serif;    color: #FFF;    height: 100%;    overflow: hidden;
}h1 {    z-index: 1000;    position: fixed;    top: 50%;    left: 50%;    transform: translateX(-50%) translateY(-100%);    font-size: 58px;    overflow: hidden;
}span {    position: relative;    display: inline-block;    animation: drop 0.75s ease 0s;
}canvas {    width: 100%;    height: 100%;
}
@keyframes drop {
    0% {        transform: translateY(-100px);        opacity: 0;
    }
    90% {        opacity: 1;        transform: translateY(10px);
    }
    100% {        transform: translateY(0px);
    }
}

var ctx = document.querySelector(&#39;canvas&#39;).getContext(&#39;2d&#39;)ctx.canvas.width = window.innerWidthctx.canvas.height = window.innerHeightvar sparks = []var fireworks = []var i = 20;while (i--) {    fireworks.push(        new Firework(Math.random() * window.innerWidth, window.innerHeight * Math.random())    )}render()function render() {    setTimeout(render, 1000 / 60)    ctx.fillStyle = &#39;rgba(0, 0, 0, 0.1)&#39;;    ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height)    for (var firework of fireworks) {        if (firework.dead) continue        firework.move()        firework.draw()    }    for (var spark of sparks) {        if (spark.dead) continue        spark.move()        spark.draw()    }    if (Math.random() < 0.05) {        fireworks.push(new Firework())    }}function Spark(x, y, color) {    this.x = x    this.y = y    this.dir = Math.random() * (Math.PI * 2)    this.dead = false    this.color = color    this.speed = Math.random() * 3 + 3;    this.walker = new Walker({        radius: 20,        speed: 0.25    })    this.gravity = 0.25    this.dur = this.speed / 0.1    this.move = function () {        this.dur--            if (this.dur < 0) this.dead = true        if (this.speed < 0) return        if (this.speed > 0) this.speed -= 0.1        var walk = this.walker.step()        this.x += Math.cos(this.dir + walk) * this.speed        this.y += Math.sin(this.dir + walk) * this.speed        this.y += this.gravity        this.gravity += 0.05    }    this.draw = function () {        drawCircle(this.x, this.y, 3, this.color)    }}function Firework(x, y) {    this.xmove = new Walker({        radius: 10,        speed: 0.5    })    this.x = x || Math.random() * ctx.canvas.width    this.y = y || ctx.canvas.height    this.height = Math.random() * ctx.canvas.height / 2    this.dead = false    this.color = randomColor()    this.move = function () {        this.x += this.xmove.step()        if (this.y > this.height) this.y -= 1;        else this.burst()    }    this.draw = function () {        drawCircle(this.x, this.y, 1, this.color)    }    this.burst = function () {        this.dead = true        var i = 100;        while (i--) sparks.push(new Spark(this.x, this.y, this.color))    }}function drawCircle(x, y, radius, color) {    color = color || &#39;#FFF&#39;    ctx.fillStyle = color    ctx.fillRect(x - radius / 2, y - radius / 2, radius, radius)}function randomColor() {    return [&#39;#6ae5ab&#39;, &#39;#88e3b2&#39;, &#39;#36b89b&#39;, &#39;#7bd7ec&#39;, &#39;#66cbe1&#39;][Math.floor(Math.random() * 5)];}function Walker(options) {    this.step = function () {        this.direction = Math.sign(this.target) * this.speed        this.value += this.direction        this.target ?            this.target -= this.direction :            (this.value) ?            (this.wander) ?            this.target = this.newTarget() :            this.target = -this.value :            this.target = this.newTarget()        return this.direction    }    this.newTarget = function () {        return Math.round(Math.random() * (this.radius * 2) - this.radius)    }    this.start = 0    this.value = 0    this.radius = options.radius    this.target = this.newTarget()    this.direction = Math.sign(this.target)    this.wander = options.wander    this.speed = options.speed || 1}