Hi Patrick,
Will this hurt your SEO? Maybe, but not in the more traditional way. For example, there is no direct penalty that this would fall under, but what it could do is confuse the user experience. Just keep in mind that SEO isn't just about your SERP positions.
As a user on any site, you want to see consistency. If you change theme part way through, then this can devalue the level of trust in the site as it no longer looks like the original site.
I would stick with one theme and if you aren't happy with the current look and feel, change your whole site to match this.
From Google's perspective, no, this wouldn't cause a devalued ranking by installing an additional theme (unless SEO issues arise from the installation), but there are a whole host of reason why you shouldn't do it.
-Andy

