Hi Morais,
We have prepared a sample that tries to meet your requirement
'Modifying selected background color'. In the sample we have edited style and
define key 'SelectionBrush' which can be modified to set different colors. We
have also provided sample for varying font color, cell background etc.
Please find the sample in below link.
SfCalendar_116294.zip
Please let us know if you need further assistance.
Regards,
Vignesh V