/* https://grabient.com/_gf2gAAgJYgiCgAAgUGgBjgBogBkgIjgAAgLB?style=linearGradient&steps=5&angle=135 */

background: linear-gradient(135deg, #00003e 0.000%, #140070 25.000%, #3f00a3 50.000%, #7600d6 75.000%, #b600ff 100.000%);

<svg xmlns="http://www.w3.org/2000/svg" width="800" height="400" viewBox="0 0 800 400" preserveAspectRatio="none">
            <!-- https://grabient.com/_gf2gAAgJYgiCgAAgUGgBjgBogBkgIjgAAgLB?style=linearGradient&steps=5&angle=135 -->
            <defs>
              <linearGradient id="gradient" x1="0.146" y1="0.146" x2="0.854" y2="0.854">
        <stop offset="0.000" stop-color="#00003e" /><stop offset="0.250" stop-color="#140070" /><stop offset="0.500" stop-color="#3f00a3" /><stop offset="0.750" stop-color="#7600d6" /><stop offset="1.000" stop-color="#b600ff" />
              </linearGradient>
            </defs>
            <rect x="0" y="0" width="800" height="400" fill="url(#gradient)" />
          </svg>

["#00003e", "#140070", "#3f00a3", "#7600d6", "#b600ff"]

// https://grabient.com/_gf2gAAgJYgiCgAAgUGgBjgBogBkgIjgAAgLB?style=linearGradient&steps=5&angle=135
// cosine gradient palette — https://iquilezles.org/articles/palettes/
const float STEPS = 5.0;
const float ANGLE = 135.0;

vec3 palette(float t) {
    vec3 a = vec3(2.038, 0.000, 0.600);
    vec3 b = vec3(2.178, 0.000, 1.286);
    vec3 c = vec3(0.099, 0.104, 0.100);
    vec3 d = vec3(0.547, 0.000, 0.705);
    return a + b * cos(6.283185 * (c * t + d));
}

// N stops sampled from the palette, blended like the CSS gradient
vec3 stops(float t) {
    float n = STEPS - 1.0;
    float x = clamp(t, 0.0, 1.0) * n;
    return mix(palette(floor(x) / n), palette(min(floor(x) + 1.0, n) / n), fract(x));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 p = fragCoord - 0.5 * iResolution.xy;
    // CSS angle: 0 = up, clockwise; span = CSS gradient-line length
    vec2 dir = vec2(sin(radians(ANGLE)), cos(radians(ANGLE)));
    float len = abs(iResolution.x * dir.x) + abs(iResolution.y * dir.y);
    float t = dot(p, dir) / len + 0.5;
    fragColor = vec4(stops(t), 1.0);
}

[
  [2.038, 0.000, 0.600],
  [2.178, 0.000, 1.286],
  [0.099, 0.104, 0.100],
  [0.547, 0.000, 0.705]
]

Gradient palette editor — #00003e to #b600ff