prefers-color-scheme设置检测系统主题色

.day   { background: #eee; color: black; }
.night { background: #333; color: white; }
@media (prefers-color-scheme: dark) {
  .day.dark-scheme   { background:  #333; color: white; }
  .night.dark-scheme { background: black; color:  #ddd; }
}
@media (prefers-color-scheme: light) {
  .day.light-scheme   { background: white; color:  #555; }
  .night.light-scheme { background:  #eee; color: black; }
}

:root {
  // Light Theme
  --PrimaryBackgroundColorLight: #f9f9f9;  
  --PageBackgroundColorLight: #efefef;
  --PrimaryColorLight: #34495e;
  --PrimaryActivedColorLight: #1890FF;
  // Dark Theme
  --PrimaryBackgroundColorDark: #293042;
  --PageBackgroundColorDark: #202634;
  --PrimaryColorDark: #c6cfd8;
  --PrimaryActivedColorDark: #1890FF;
}

.day.light {
  background-color: var(--PrimaryBackgroundColorLight);
}

const themeMedia = window.matchMedia("(prefers-color-scheme: light)");
if (themeMedia.matches) {
  // 日间模式
} else {
  // 夜间模式
}

const themeMedia = window.matchMedia("(prefers-color-scheme: light)");
themeMedia.addListener(e => {
  if (e.matches) {
    console.log("light");
  } else {
    console.log("dark");
  }
});