riot-web/res/themes/light/css/_mods.scss

// sidebar blurred avatar background
//
// if backdrop-filter is supported,
// set the user avatar (if any) as a background so
// it can be blurred by the tag panel and room list

@supports (backdrop-filter: none) {
    .mx_LeftPanel {
        background-image: var(--avatar-url, unset);
        background-repeat: no-repeat;
        background-size: cover;
        background-position: left top;
    }

    .mx_GroupFilterPanel {
        backdrop-filter: blur($groupFilterPanel-background-blur-amount);
    }

    .mx_LeftPanel .mx_LeftPanel_roomListContainer {
        backdrop-filter: blur($roomlist-background-blur-amount);
    }
}

.mx_RoomSublist_showNButton {
    background-color: transparent !important;
}

a:hover,
a:link,
a:visited {
    text-decoration: none;
}