There are thousands of WordPress themes you can use for your website according to your niche.
In fact, there are presently 3990 WordPress.org (WordPress.com is different) free themes you can choose from. It is not necessary to use a paid theme.
However, the biggest disadvantage of using a free theme is you have only limited features in which you won’t be able to change the theme color or customize the settings according to your desire!
You sure can make some tweak here and there but you will not have full control of all the settings or change everything according to your wish.
Another disadvantage of using the free theme is, the theme name will appear in the footer section of your site, with an exception few themes in which you can change the footer content.
The greatest advantage of using a Pro theme is, you can customize everything according to your wish! There is no limitation, whatsoever!
Here is the list of the most easily customizable and professional themes you can choose for your website. They are all compatible with all WordPress plugins and you can try them out for free too.