message: feat: add dark mode basic variables
diff: -:root {
  +body {
  // Color Palette
  --van-black: #000;
  --van-white: #fff;
  --van-radius-lg: 8px;
  --van-radius-max: 999px;
  }
  +
  +.van-theme-dark {
  + --van-text-color: #f5f5f5;
  + --van-text-color-secondary: #707070;
  + --van-text-color-tertiary: #4d4d4d;
  + --van-border-color: #3a3a3c;
  + --van-active-color: #3a3a3c;
  + --van-background: #000;
  + --van-background-light: #2c2c2e;
  +}
  