Example to change default fog white to custom fog black...

bx_menu_popup('sys_switch_language_popup', window, {fog:{color:'#000',opacity:0.7}});

bx_menu_popup('sys_switch_template', window, {fog:{color:'#000',opacity:0.7}});

Those popups have same effect. Please, make it so in Copyright.

Have tried in Custom Styles...

.bx-popup.fog {
background-color: rgba(0, 0, 0, .7);
}

Still doesn't work.