I think themes matter a lot. Themes are your environment. I would encourage anyone creating their new web site, after carefully choosing your niche, to really take your time, consider a few themes, and choose one that matches your niche.
It's your environment. So, if you are talking about being grounded, then a theme that looks like it is floaty on in the clouds doesn't quite go with it. If you have a children's niche, then something associated with children goes well. If food, then something tasty or that evokes taste may work. If a theme looks industrial, does it really go well with a niche about a path to personal spiritual health?
Also, one or more graphics help a great deal. Sometimes we use the same themes but changing the graphic customizes and personalizes it.
So absolutely while making a great about me, finding effective headings and menus and topics, the theme is a big part of your site as well as the graphics you may find to use on your theme.