NEZ-1812 style：修改icon下拉菜单样式

nezha-fronted/src/assets/css/components/common/rightBox/chartRightBox/chartRightBox.scss

@@ -148,6 +148,24 @@
         padding-left: 10px;
         box-sizing: border-box;
       }
+      .mapping-iconSelect {
+        .el-input__inner{
+          color: transparent;
+        }
+        .el-input__inner::-webkit-input-placeholder {    /* Chrome/Opera/Safari */
+          color: $--color-text-placeholder;
+        }
+        .el-input__inner::-moz-placeholder { /* Firefox 19+ */
+          color: $--color-text-placeholder;
+        }
+        .el-input__inner:-ms-input-placeholder { /* IE 10+ */
+          color: $--color-text-placeholder;
+        }
+        .el-input__inner:-moz-placeholder { /* Firefox 18- */
+          color: $--color-text-placeholder;
+        }
+      }
+      
       .mapping-icon{
         position: absolute;
         left: 8px;

nezha-fronted/src/components/common/rightBox/chart/chartConfig.vue

@@ -675,6 +675,7 @@
                       style="width: 75px"
                       :placeholder="$t('el.select.placeholder')"
                       @change="change('valueMapping', index)"
+                      class="mapping-iconSelect"
                     >
                     <template slot="prefix">
                       <i class="mapping-icon" :class="item.icon" :style="{ color: item.color.icon }"></i>

nezha-fronted/src/components/common/rightBox/chart/systemChartConfig.vue

@@ -637,6 +637,7 @@
                       style="width: 75px"
                       :placeholder="$t('el.select.placeholder')"
                       @change="change('valueMapping', index)"
+                      class="mapping-iconSelect"
                     >
                     <template slot="prefix">
                       <i class="mapping-icon" :class="item.icon" :style="{ color: item.color.icon }"></i>