The dress code seems largely to have been replaced by specifying dress. So, 'formal' after 7pm has been replaced by 'white tie', and 'semi-formal' by 'black tie' and so on.
Your hosts have stuck wit the code, so go with it: casual = jacket and tie (so you have great freedom on shirt colour, tie design and trousers). As Airborne Artist said, do this, do a recce, and then adjust.
Batco
PS. The drawback of ditching the code for specifying the dress is that it usually only specifies what gentlemen should wear and the ladies then get away with virtually anything again.