Template:Info/styles.css

.infobox.mw-collapsed {
    background-color: transparent;
    box-shadow: inset 0 0 0 1px var(--border-color-base);
}

.infobox__snippet.mw-collapsible-toggle {
    /* Disable default style */
    float: none;
    border-top-left-radius: var(--border-radius--medium);
    border-top-right-radius: var(--border-radius--medium);
}

.infobox__snippet.mw-collapsible-toggle.mw-collapsible-toggle-collapsed {
    border-bottom-left-radius: var(--border-radius--medium);
    border-bottom-right-radius: var(--border-radius--medium);
}

.infobox__snippet.mw-collapsible-toggle:hover {
    background-color: var(--background-color-quiet--hover);
}

/* Might as well steal from skin */
.infobox__snippet.mw-collapsible-toggle .mw-ui-icon-wikimedia-collapse {
    width: var(--font-size-small);
    height: var(--font-size-small);
    flex-shrink: 0;
    transition: var(--transition-menu);
    transition-property: transform;
}

/* Might as well steal from skin */
.infobox__snippet.mw-collapsible-toggle.mw-collapsible-toggle-collapsed
    .mw-ui-icon-wikimedia-collapse {
    transform: rotate(180deg);
}