Hi @chairool,

You can use this CSS code:

#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content p {line-height:18px}

The CSS codes should be added in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Styles 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.    

Note: the red marked code can be changed as you like. 

Hi, I've tried and waited for couple days and still not working. Then I added !important and I think it may bypass the theme coded and it works!

#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content p {line-height:18px !important}