fix cascader style bug (#3877)

packages/cascader/src/main.vue

@@ -319,6 +319,7 @@ export default {
       if (this.disabled) return;
       if (this.filterable) {
         this.menuVisible = true;
+        this.$refs.input.$refs.input.focus();
         return;
       }
       this.menuVisible = !this.menuVisible;

packages/theme-default/src/cascader.css

@@ -7,13 +7,10 @@
   @b cascader {
     display: inline-block;
     position: relative;
-    background-color: transparent;
 
     .el-input,
     .el-input__inner {
       cursor: pointer;
-      background-color: #fff;
-      z-index: var(--index-normal);
     }
 
     .el-input__icon {