nextcloud-server/apps/theming/css/settings-admin.scss

#theming {
    input {
        width: 230px;
    }

    input:focus,
    input:active {
        padding-right: 30px;
    }

    .fileupload {
        display: none;
    }

    div > label {
        position: relative;
    }

    .theme-undo {
        position: absolute;
        top: -7px; // input padding
        right: 4px; // input right margin + border
        cursor: pointer;
        opacity: .3;
        padding: 7px;
        vertical-align: top;
        display: inline-block;
        visibility: hidden;
        height: 32px; // height of input
        width: 32px; // height of input
    }
    form.uploadButton {
        width: 411px;
        display: flex;
        align-items: center;
    }
    form .theme-undo,
    .theme-remove-bg {
        cursor: pointer;
        opacity: .3;
        padding: 7px;
        vertical-align: top;
        display: inline-block;
        float: right;
        position: relative;
        top: 4px;
        right: 0px;
        visibility: visible;
        height: 32px;
        width: 32px;
        // right align
        margin-left: auto;
    }
    form .theme-undo:not([style*="display:"]) ~ .theme-remove-bg {
        // Only align the undo button if both are shown
        margin-left: 0;
    }

    input[type='text']:hover + .theme-undo,
    input[type='text'] + .theme-undo:hover,
    input[type='text']:focus + .theme-undo,
    input[type='text']:active + .theme-undo,
    input[type='url']:hover + .theme-undo,
    input[type='url'] + .theme-undo:hover,
    input[type='url']:focus + .theme-undo,
    input[type='url']:active + .theme-undo{
        visibility: visible;
    }

    label span {
        display: inline-block;
        min-width: 175px;
        max-width: 175px;
        white-space: wrap;
        padding: 8px 0px;
        vertical-align: top;
    }

    .icon-upload,
    .uploadButton .icon-loading-small {
        padding: 8px 20px;
        width: 20px;
        margin: 2px 0px;
        min-height: 32px;
        display: inline-block;
    }

    #theming_settings_status {
        height: 26px;
        margin: 10px;
    }

    #theming_settings_loading {
        display: inline-block;
        vertical-align: middle;
        margin-right: 10px;
    }

    #theming_settings_msg {
        vertical-align: middle;
        border-radius: 3px;
    }

    #theming-preview {
        width: 230px;
        height: 140px;
        background-size: cover;
        background-position: center center;
        text-align: center;
        margin-left: 178px;
        margin-top: 10px;
        margin-bottom: 20px;
        cursor: pointer;
        background-color: var(--color-primary-default);
        background-image: var(--image-background-default, var(--image-background-plain, url('../../../core/img/app-background.jpg'), linear-gradient(40deg, #0082c9 0%, #30b6ff 100%)));

        #theming-preview-logo {
            cursor: pointer;
            width: 20%;
            height: 20%;
            margin-top: 20px;
            display: inline-block;
            background-position: center;
            background-repeat: no-repeat;
            background-size: contain;
            background-image: var(--image-logo, url('../../../core/img/logo/logo.svg'));
        }
    }

    .theming-hints {
        margin-top: 20px;
    }

    .image-preview {
        display: inline-block;
        width: 80px;
        height: 36px;
        background-position: center;
        background-repeat: no-repeat;
        background-size: contain;
    }

	#theming-preview-logoheader {
        // Only using --image-logoheader to show the custom value only
        background-image: var(--image-logoheader);
    }

	#theming-preview-favicon {
        background-image: var(--image-favicon);
    }

    #user-theming {
        margin-top: 44px;
        display: flex;
       & > div {
            max-width: 400px;
            margin-bottom: 44px;
        }
    }
}

/* transition effects for theming value changes */
#header {
    transition: background-color 500ms linear;
    svg, img {
        transition: 500ms filter linear;
    }
}