When Spanish.SE graduates, it will get a dedicated design. Until then, you're stuck with the generic beta theme.
Reaching graduation represents a community achievement. But in order to remove the beta label, we need to have confidence that a site is self-sustaining. One of the things we look at is the question rate. When a site is able to sustain a strong rate of questions (and several other indicators of a strong community) it goes to our designers for a custom theme.
In the meantime, there are a few things you can do to improve the odds of graduation. One of my favorite ideas is a weekly topic challenge. A small number of users working together on meta can spark new life in a site by committing themselves to asking questions is a specific tag or about a specific topic.