fix: 优化暗色背景下禁用按钮样式 (#1663)

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#### Summary of your change

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- [ ] Made sure tests are passing and test coverage is added if needed.
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frontend/src/styles/element-dark.scss

@@ -306,4 +306,7 @@ html.dark {
         background-color: rgb(56, 59, 59);
         color: var(--el-color-warning);
     }
+		.el-dropdown-menu__item.is-disabled {
+			color: var(--panel-button-disabled);
+	  }
 }