It is important to remember that ‘Vertical Tabs’ is still an experimental feature and isn’t very old.
I would suspect that customization is something that can be expected in the future, however, functionality is usually the primary goal of experimental features. Cosmetic appeal is definitely something that would be more along the lines of features that are stable and ready for production.
You should be able to customize the tabs now through chromium CSS edits, but that is definitely going to require some advanced knowledge of chromium itself as well as CSS.
The cosmetics of vertical tabs has been comment on quite a bit, so I am sure it will be looked into. The amount of coding and effort that is going to be required in order to enable themes or even just basic customizations is quite intense. I would venture to guess that creating a system to customize vertical tabs it probably going to be a bigger project than vertical tabs is as a whole.