Set active color and inactive color of legend in dark theme

[wangtao0101]

• I have searched the issues of this repository and believe that this is not a duplicate.

Reproduction link

Steps to reproduce

在黑色主题下，legend active的时候颜色有点暗，而inactive比较亮。希望能够改成active的时候亮一点，inactive的时候灰一点。
或者能够提供自定义的api

Environment	Info
g2	4.1.14
System	-
Browser	-

[visiky]

和主题进行一个 merge 设置一下主题：

theme: {
   components: {
          legend: {
            top: {
              itemStates: {
                unchecked: {
                  nameStyle: {
                    fill: '#666',
                  },
                  markerStyle: {
                    stroke: '#666'
                  },
                },
              }
            }
          }
        }
}