Code Sketch

size(935, 645)
clear()
setSpeed(fast)
val cb = canvasBounds
initRandomGenerator(-376991280140970457L)

def backdropFill = cm.linearMultipleGradient(
    0, 0, 0, cb.height,
    Seq(0, 0.6, 0.7, 1),
    Seq(
        ColorMaker.hsl(50, 0.8, 0.1),
        ColorMaker.hsl(60, 1, 0.50),
        ColorMaker.hsl(30, 1, 0.50),
        ColorMaker.hsl(215, 0.7, 0.50),
    ),
    false
)

def backdrop = Picture.rectangle(cb.width, cb.height)
    .withPenColor(noColor)
    .withFillColor(backdropFill)
    .thatsTranslated(-cb.width / 2, -cb.height / 2)

def mountains = Picture {
    setPenColor(noColor)
    repeatFor(0 to 3) { i =>
        // each t is less than the one in the previous iteration
        val t = math.pow(1 - i / 5.0, 2)

        // each closer/newer hill is darker
        val r0 = t * 0.20 + 0.05
        val g0 = t * 0.16 + 0.03
        val b0 = t * 0.10 + 0.02

        val r = (r0 * 255).toInt
        val g = (g0 * 255).toInt
        val b = (b0 * 255).toInt

        // there are more undulations in closer/newer hills
        val height = 6 + i * 3

        val x0 = cb.x

        // each closer/newer hill is lower down
        val y0 = cb.height / 2 - 200 - i * 60

        setPosition(x0, y0)
        setFillColor(cm.rgb(r, g, b).lighten(0.1))

        var y = y0

        // to smoothen out the hill undulations
        var prevRandom = 0

        for (x <- rangeTill(cb.x, cb.x + cb.width, 10)) {
            val currRandom = random(-height, height)
            y = y + currRandom * 0.5 + prevRandom * 0.5
            prevRandom = currRandom
            lineTo(x, y)
        }

        lineTo(cb.x + cb.width, y)
        lineTo(cb.x + cb.width, cb.y)
        lineTo(cb.x, cb.y)
        lineTo(x0, y0)
    }
}

def sunRadius = 60

def sunFill = cm.radialMultipleGradient(
    0, 0, 60,
    Seq(0, 0.4, 0.5, 1),
    Seq(
        ColorMaker.hsl(60, 1.00, 0.95),
        ColorMaker.hsl(40, 1.00, 0.85),
        ColorMaker.hsla(30, 1.00, 0.85, 0.2),
        ColorMaker.hsla(30, 1.00, 0.80, 0.05)
    )
)

def sun = Picture.circle(60)
    .withPenColor(noColor)
    .withFillColor(sunFill)

def drawing = picStack(
    backdrop,
    sun.thatsTranslated(cb.width / 6.3, cb.height / 7),
    mountains
)

draw(drawing)