Hi Luca!
The problem with your suggestion is that it is potentially too far ranging. There are other objects in Moodle to which the .side, .picture and .side classes can be applied.
If Jonathan only wants to change the color background of those classes within the context of the calendar "main page", i.e. the daily view, then the following classes should be used:
#calendar .maincalendar .eventlist .topic {...}
#calendar .maincalendar .eventlist .picture {...}
#calendar .maincalendar .eventlist .side {...}
Hope that helps,
Joseph