That is one way to check if it’s the only element, being the first and last child at the same time…

        &.o-inline {
            &:where(:has(i:first-child:last-child)) {
                opacity: var(--o-mail-ActionList-Button-opacity);

    & button.o-simulateDarkTheme.o-hasBtnBg:not(.o-tag-DANGER):not(.o-tag-SUCCESS):not(.o-tag-PRIMARY) {
        background-color: #{$gray-800};
        border: 1px solid rgba(white, 0.1);

        &.active {
            // Same colors than dark theme since dark theme is simulated for call view.
            background-color: #17373b !important;
            border-color: #03f9e3 !important;
        }
    }