Auto dark-mode with option to set it individually

It would be nice to have an auto-dark mode for element.io so people like me don't burn their eyes when they visit the page. This can be solved pretty easily over a CSS stylesheet that has the media type `prefers-color-scheme: dark`, for example

```
@media (prefers-color-scheme: dark) {

}
```

Also a toggle to enable dark mode for each system individually would be nice to fix issues for some Linux desktops not supporting system-wide darkmode for browsers.