Set active color and inactive color of legend in dark theme #3375

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'
                  },
                },
              }
            }
          }
        }
}