Themes as in colours or other stuff (for want of a better word)?
You could have a "Moulin Rouge" style wedding - I've seen some fab cakes around which have this theme. Loads of feathers and cleavage

A few people have had James Bond themed weddings (bond girl/villain table names, Aston Martin car to take Bride to reception etc)
Gothic.... my hairdresser is having a goth wedding (well, she's a diehard goth) with a black taffeta two piece dress and a Nightmare before Christmas cake from Chochywadydodah or whatever its called!
Medieval - in a lovely old castle with rich jewel toned dresses and decor (we were going to do this but I thought I'd be worried about smashing things all day long)
Nautical/South Pacific - you could get married on a moored ship (the SS Great Britain in Bristol is a great example) and do something imaginative with favours etc. Mind has gone blank here!
Flower themes - you could base everything around roses (v.expensive) or tulips for a spring wedding and perhaps give out potted tulip bulbs (in various colours) as favours. You could also tie labels on the pots so that they can be used for place names.
Lord of The Rings/Elvish themes seem to be becoming quite popular.... you could have a humanist ceremony in a forest glade with flowers strewn over the grass and a garland of flowers in your hair (not wildflowers as picking them seems to have become illegal)...
For colour based themes....
Silver and lilac
Black and white (seems to be popular if the wedding mags have any say in it!!)
Red and white - seems to suit both summer and winter but I prefer it for winter!
Shades of pink.... from hot pink to palest pink. This would look great if you have quite a few bridesmaids - you could have the eldest/CBM in hot pink and a little flowergirl in palest pink and the rest wearing shades somewhere between!! Masses of pink flowers (again in various shades) set off with ferns and ivy would make for a very romantic and sumptuous feel. I wouldn't go as far as pink food though!!
Shades of gold or silver
Bronze and orange - very autmnal colours.
We're having a mixture of themes.... our colours are pale gold/champagne with apricot and pale pink and we're having masses of roses in those colours. Table decorations will also have napkins tied with a rose on them for the ladies and just tied with ivy for the blokes. I'm looking forward to seeing it all coming together!
Hope this helps a bit,