Add widescreen CSS for themes

also moves some declarations into the new CSS class "widescreen-separate".
dos2unix two remaining files

charts/index.html and css/widescreen.css were still using DOS style line endings (CRLF).
Add themes event handler.

The event handler is general: If the rules explained in the files are followed, additional themes can be added without any javascript changes!
Fix a bug when removing a theme.

I was counting upwards in a loop that started on the last element :)
This worked as long as the link to remove was the last one, but caused the browser to hang otherwise.
This could be reproduced by reloading the page while in dark theme. Firefox would keep the checkbox checked, but the theme wouldn't have been loaded. (This still needs to be fixed; it just doesn't crash the browser now.)