You need to change several files :
/usr/syno/synoman/webman
you should change the favicon.ico
file./usr/syno/synoman/webman/resources/images
you should change all the icon_dsm*.png
files :icon_dsm.png
icon_dsm_16.png
icon_dsm_32.png
icon_dsm_48.png
icon_dsm_64.png
icon_dsm_96.png
Reference :
http://www.nas-forum.com/forum/topic/46223-m/?do=findComment&comment=1319264941