Your invitation is one of the first indications your guests may have about your wedding. Although you don't have to, it's fun to make an extra effort when picking out your invite so that it matches the theme of your wedding or reception.
But more importantly, don't pick an invitation that is nothing whatsoever like the theme of your wedding. For example, no matter how cute it is, it would be strange to have a carnival-themed invitation to a simple wedding at a yoga retreat.
When in doubt, just stick to a simple invitation that has a hint of your wedding colors.