There is probably an easier way to create a gradient skybox but I am lazy.

Simple Sky Shader

const skygeometry = new SphereGeometry( 25, 50, 50 , 0, 2 * Math.PI);
const skymaterial = new MeshBasicMaterial();
skymaterial.side = BackSide;
skymaterial.onBeforeCompile = function (shader) {
    shader.vertexShader = shader.vertexShader.replace('#include <common>', '#include <common>\n#define USE_UV');
    shader.fragmentShader = shader.fragmentShader.replace('#include <common>', '#include <common>\n#define USE_UV');
    shader.vertexShader = shader.vertexShader.replace('#include <uv_vertex>', `
        vUv = ( uvTransform * vec3( uv, 1 ) ).xy;
    `)
    shader.fragmentShader = shader.fragmentShader.replace('#include <map_fragment>', `
        vec4 col1;
        vec4 col2;
        float mixAmount;
        if (vUv.y > 0.5) {
            col1 = vec4( 249, 229, 180, 1 ) / 255.0;
            col2 = vec4( 0, 57, 115, 1 ) / 255.0;
            float newY = (vUv.y - 0.5) * 2.0;
            mixAmount = sqrt(newY)*2.0;
        } else {
            col1 = vec4(0.6);
        }
        diffuseColor *= mix(col1, col2, mixAmount);
    `);
};
const skysphere = new Mesh( skygeometry, skymaterial );
skysphere.name = 'skysphere';
scene.add( skysphere );