element-web/res/css/structures/_SplashPage.pcss

/*
Copyright 2022 The Matrix.org Foundation C.I.C.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

.mx_SplashPage {
    position: relative;
    height: 100%;

    &::before {
        content: "";
        display: block;
        position: absolute;
        z-index: -1;
        opacity: 0.6;
        background-image: radial-gradient(
                53.85% 66.75% at 87.55% 0%,
                hsla(250deg, 76%, 71%, 0.261) 0%,
                hsla(250deg, 100%, 88%, 0) 100%
            ),
            radial-gradient(41.93% 41.93% at 0% 0%, hsla(222deg, 29%, 20%, 0.28) 0%, hsla(250deg, 100%, 88%, 0) 100%),
            radial-gradient(100% 100% at 0% 0%, hsla(250deg, 100%, 88%, 0.174) 0%, hsla(0deg, 100%, 86%, 0) 100%),
            radial-gradient(106.35% 96.26% at 100% 0%, hsla(250deg, 100%, 88%, 0.4) 0%, hsla(167deg, 76%, 82%, 0) 100%);
        /* blur to reduce color banding issues due to alpha-blending multiple gradients */
        filter: blur(8px);
        inset: -9px;
        mask:
            /* mask to dither resulting combined gradient */
            url("$(res)/img/noise.png"),
            /* gradient to apply different amounts of dithering to different parts of the gradient */
                linear-gradient(
                    to bottom,
                    /* 10% dithering at the top */ rgba(0, 0, 0, 0.9) 20%,
                    /* 80% dithering at the bottom */ rgba(0, 0, 0, 0.2) 100%
                );
    }
}