Well I know the basics of CSS so my theme is heavily customized off the default theme. The mobile theme is heavily customized off of a free modernized GoMobile theme. So I don't use a free theme however I've made my own themes based off of free themes.
Not currently (my forum is using a custom theme), but when my forum was just starting up I used free themes that I customized to suit the overall feel/theme of the forum better. I don't think it is necessary to spend for a paid theme right away when there are some really good free themes out there. With how often forums close it's best to wait a while.
It will be better for your that if you properly buy themes for your forum, because free themes have danger of virus and other defects for your website.
TheBitZone is also powered by MyBB. I am more of a backend developer and hence I develop some custom plugins myself - we do also have free plugins installed. As for themes and templates, we use a free theme by iAndrew and modify it to fit our niche - general Bitcoin and Crypto discussion.
We actually contacted a few designer before we open the forums as we would like to have a custom theme made. However, most of the designers price is just too high. As a pay-to-post forums, we cannot afford to waste our money when we can create ourselves or get it for free.
As time goes on, I will learn more about creating themes for MyBB and write my own custom themes. I've designed custom themes for Wordpress and templates for Joomla before. MyBB is a forums and hence it do not work the same.
I don't mind doing using free themes for software that are free like phpBB or MyBB but for Xenforo or IPB I'd go with a paid theme as it's a paid software