Hi and welcome to the site!

It is perfectly normal for the car not to have stripes and yes you can add them at a later date. I bought my performance blue ST 'naked' and fitted side dtripes a few months ago. I don't think the dealers do it after purchase and if they do it will cost you bucks! I did mine myself and it has turned out 'ok'. A lot of the owners here recommend Dave at DMB and he will fit you sides for around £25 supplied and fitted. Not sure where he is based and if he would be local to you, but a sign writing place should be able to sort you out.