How to add border radius to all elements?

Use following CSS:

.box-blog-entry .box-blog-thumb a img, .group-box .group-box-image img, .product .product-thumbnail img {
    border-top-left-radius: 6px!important;
    border-top-right-radius: 6px!important;
}

.box-blog-entry .box-blog-details, .box-blog-entry .group-box-bottom, .product .woocommerce-listing-item-content {
    border-bottom-left-radius: 6px!important;
    border-bottom-right-radius: 6px!important;
}

main#content .thumbnail, aside#left .author-bio img.avatar, .comment-avatar img, #submit, .shortcode-login, .dd_single-event-image img, .offers-container .offer, .buddypress #members-list li .item-avatar, #sidebar .rtmedia-container ul.rtmedia-list li.rtmedia-list-item div.rtmedia-item-thumbnail img, #sidebar .bp-user-groups li img, #sidebar .bp-user-friends li img, .widget_bp_core_recently_active_widget .avatar-block .item-avatar img, .widget_bp_core_whos_online_widget .avatar-block .item-avatar img, .gallery-icon img, .header-top-login, .header-top-signin, .single-headers #item-header-avatar a img, #item-header-content .member-header-actions .login-popup-action, .single-headers#item-header .group-button, textarea, #sidebar .bp-profile-fields .item.bg, img.avatar { 
    border-radius:6px!important; 
}