I feel like this is an age-old question for webmasters. In my opinion, a paid theme is not necessary. When possible I would always go the free route when starting off and customizing the theme to fit your needs. For example, I took my forum and used the Xenforo default theme but customized the color scheme to fit what I wanted.
I may put out some money for a theme somewhere down the road but it doesn't seem like an absolutely necessary item when trying to get a community started.
I have the same question in mind, lets assume we start off with a polished theme and logo but no user joins in it will be a waste of money. First start with a free theme, modified slightly and as users join us and it becomes active we can go for a paid theme.
I don't mean for it to sound like someone who's being a cheapskate but usually, these hobby sites don't have huge budgets. I think you should still be making sure that you are investing a lot of time into it and make it look like you give a crap about the website you started. I just don't think that someone should feel like that MUST have a paid theme cause paid theme = a lot of new users.
Not sure if they add in mobile ability sometimes. For instance, the IO/Dark IO themes (XenForo) from Theme House claims to be more mobile optimized - but they mean as far as an Apple mobile is concerned (XenForo is mobile optimized out of the box.). Well, is that important? Not sure - but I bought a couple anyway cause they look cool.
I have seen vbulletin forums using the default theme be active as hell. I have seen forums with a crazy custom theme have absolutely no activity. It is all based on the efforts of the staff team to keep the forum entertaining and full of joy for the members. Looks are only an extra.
I personally don't see a point in paying someone else to make a theme for you if you can make or edit one to your liking without having to go through a middleman. I've used custom (non-default) themes over the years but edited them to my liking (so much in recent years that it's almost an entirely different theme!). Never paid.
