fix: chart.theme as unrequired params

[pepper-nice]

描述

theme 属性支持在 chart.options() 里申明：

const chart = new Chart({ container });

chart.options({
  theme: 'classic',
  type: 'line',
  clip: true,
  data: {
    type: 'fetch',
    value: 'data/aapl.csv',
  },
  encode: {
    x: 'date',
    y: 'close',
  },
});

chart.render();