I think that because of the empty Custom CSS textarea. This font-size option is also custom CSS, but when the textarea is empty it doesn't update the dynamic CSS file. We'll take a look on this and fix this logic.

Try to add  /* test */ in textarea, then save it and do Ctrl+F5 on forum front.

In any case this CSS code may helpful:

#wpforo #wpforo-wrap .wpforo-post .wpf-right .wpforo-post-content p {
   font-size: 24px;
    line-height: 30px;
}

Thanks. Editing the CSS code did the trick. And I didn't realize that Ctrl+F5 was needed. I just thought a simple refresh of the page would work. But, indeed, Ctrl+F5 is needed.

Not related to your question but just a suggestion to help. I checked your site and you have a lot of inline images/pictures.

Install this

https://wordpress.org/plugins/ari-fancy-lightbox/

in order to open inline images into a lightbox. Works great with wpForo and your site will look even more professional.

Thanks for the suggestion @Anonymous20. I'll add it to my to do list.