Use the CSS code below:

#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content p, #wpforo #wpforo-wrap .wpforo-revision-body p{
    line-height: 20px !important;
}

Put the code in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Style Tab.  

Please don't forget to delete all caches and press Ctrl+F5 (twice) on the frontend before checking to reload the updated CSS file.   

The red marked code can be changed as you like.

please leave some example URL.

On my forum the line spacing somehow is very big. There's much more space between the lines than on this official WPForo. Could you help me out? https://prnt.sc/sbdpdu

This is what it looks like while editing: https://prnt.sc/sbdqox

- Gedaante