DS1's party was just family, mostly, and didn't so much have a theme. I found printable invitation cards that had a border that looked like bubbles (blue and green circles). I also found a Richard Scarry picture that was a little pig blowing bubbles -- so I guess our theme was bubbles!
It's actually a cute theme if you've got other little ones coming, esp. for summer -- DS1's birthday is in January so we didn't do this, but it'd be cute to have a bubble machine or bubbles as favors.
I thought it was appropriate because that was the first word he could say that we could really recognize.
I just bought plain colored paper plates/cups/napkins/balloons in shades of blue and green -- the invitations were the only thing that were really 'themed'.