select 下拉图标实心样式

select{
   background-image:
        linear-gradient(45deg, transparent 50%, #9A9A9A 50%),
        linear-gradient(135deg, #9A9A9A 50%, transparent 50%);
    background-position:
        calc(100% - 20px) calc(1em - 2px),
        calc(100% - 15px) calc(1em - 2px),
        100% 0;
    background-size:
        5px 5px,
        5px 5px,
        2.5em 2.5em;
    background-repeat: no-repeat;
   }