So, do you think it is worth the effort to create a free WordPress or Tumblr theme in order to promote your business?

Why not? Especially if your portfolio in not too big.

Just my opinion but I don't think you'd get much traction from it. Why not create a theme for themforest and sell it, this way you're promoting still but getting a little back from all those hours put in.

I was think about offering it to some place like Smashing Magazine, since it seams to me that those sites attract a finer clientele than stock sites. But I still first have to design he theme

I think yes ! It will help a lot for you to get the attention of the viewer to visit your site.

Great idea... If you make them optin to your mailing list to receive it you would then be able to market to them later with further services that you want to offer.