First of all remember that sub-domains are considered by the search engines as completely new domains different from the main domain. So you do not get any of the SEO benefits including Page Rank, Link Juice, etc. It is better to have a separate domain if you want to build a new site unrelated to the main site. SEO benefits would be almost the same either wise.
As for your plan of a new topic, it is way better from SEO perspective to have niche specific sites and blogs. I am talking of this scenario.
If you have Google Adsense on your website or ads from third parties, it is imperative that your niche is specific on a particular domain. WordPress themes and startups & finance are two diametrically opposite marketplaces and mixing these two up would be harmful rather than beneficial for you.
That being said, you had better buy a new domain for WordPress themes. Of course, you would have to do all the SEO from the scratch but it would go a long way for you at the end of the day. You would have to do that in any case. Good Luck!