You shouldn't use a lot of themes on your forums and keep changing them regularly. It annoys the users of the forum and even you will lose traffic. The best practice would be to research about different themes online. After that choose the best theme according to your requirements. . Stick to that themes until you build a strong reputation among members and have a lot of members on the forum. But there is no strict limitation to this, just you need to buy a good hosting. The more quality hosting you have, you can sue the more themes on your forum.
I believe you should choose a theme wisely, test on your site and stick with the one that suits you best. You should not use another theme unless you want to update your site. Theme gives identity and don't mess up with your site identity.