theme: fix theme file path parsing issue on windows.
theme: fix theme file path parsing issue on windows.

':' is usually regarded as the delimetr for new theme path
but it uses to look up the drive, ie, c: on windows, on the other hand.
this patch fixes to determine wheter ':' is used for drive name or delimeter.

@fix

tasn/elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary