I'd suggest you read this article:
www.limesurvey.org/manual/New_Template_System_in_LS3.x
If you need a professional to redesign a bit the themes (colors, fonts, size, etc), any designer with bootstrap experience will be fine. You can even use some bootstrap theme generators:
getbootstrap.com/docs/3.3/customize/
pikock.github.io/bootstrap-magic/3.0/
www.bootstrap-live-customizer.com
If now you want more advanced changes, like new structure of pages, different menu/index, etc, you need someone with twig knowledge.
twig.symfony.com
Finally, if you want to modify or create theme's options, you need someone knowing JavaScript.
So a frontend designer with good knowledge of Bootstrap, Twig and JavaScript should be able to do any customization you want, even if he has no limesurvey knowledge at all.