className样式被上层css样式覆盖

#### 描述问题：

button按钮的className样式被上层css样式覆盖导致不生效，是否有类似css中“!”的写法提高class的优先级？

{
"type": "button",
"level": "primary",
"label": "提交"
},
{
"type": "button",
"level": "danger",
"className": "ml-40", //不生效
"label": "退出"
}

#### 截图或视频：


#### 如何复现（请务必完整填写下面内容）：

1. 你是如何使用 amis 的？
`sdk`

2. amis 版本是什么？请先在最新 beta 版本测试问题是否存在
sdk 1.8

4. 粘贴有问题的完整 `amis schema` 代码：
```
code here...
```

Start by reproducing the issue with the SDK button schema shown, using the `className` value `ml-40` and the reported amis SDK 1.8 environment. No source file or test is identified in the report; done means the button's custom class styling remains effective when an upper-level CSS rule also applies.

Written by the indexing model from the issue text.