backgroundColor:'none'不能正常消除背景色

[TianXinX]

One-line summary [问题简述]

使用dark主题，默认具有背景颜色 更新到echarts4后 配置backgroundColor:'none' 背景色为纯黑色 更新前使用版本3.7.0，可以正常消除背景颜色

Version & Environment [版本及环境]

• ECharts version [ECharts 版本]: 4.0.4
• Browser version [浏览器类型和版本]: Chrome 63.0.3239.132
• OS Version [操作系统类型和版本]: Windows10

Expected behaviour [期望结果]

希望可以支持backgroundColor:'none'的情况下无背景色

ECharts option [ECharts配置项]

option = {
   backgroundColor:'none',
    xAxis: {
        type: 'category',
        data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    },
    yAxis: {
        type: 'value'
    },
    series: [{
        data: [820, 932, 901, 934, 1290, 1330, 1320],
        type: 'line'
    }]
}

Other comments [其他信息]

[100pah]

可设为 null 清除颜色。

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