There are plenty of really NICE trims available, from only around £6-99 a set! Most cheap "Discount Hardware stores" sell 'em off in huge piles, or try your local small Motor Accessory shop.
A nice set of original Citroen trims went on eBay only last week, for less than a tenner!!
Personally, I think Citroen standard steel wheels look horrible.....so either cover 'em up quick...or keep an eye on eBay for a good set of Alloys that will fit. (Pug 205 GTi 1.9 ones look well nice, also the earlier Pug 205 GTi "pepperpot" from the 1.6., and they will both go straight on without spacers etc. You would just need to get hold of the correct flat-seat (with washer) bolts which are a little longer.
IIRC, a couple of Club Members are currently looking to sell decent sets of alloys, for not a lot of money, which would smarten up your BX no end! Check out the "For Sale / Wanted" section of this Forum!
Good Luck!!
Dave