Hey joon,
The line height is coming from the reset file which gives the theme it’s base class rules. However, the line height is dictated by a variable found in the theme’s overlay.
If you are using for example ‘default’ theme with the ‘ice’ overlay, you would navigate to your forum-themes > default > styles > overlays folder, and open ‘ice.php’
On line 26 you will see
$mainLineHeight = '1.2em';
This can be changed to whatever value you like. For an example ‘1.3em’ or ‘1.4em’ might better suit your larger font size. You can also use any standard CSS value for line-height such as ‘4px’ or ‘5%’.
Since it looks like you are using a custom theme, your theme name, and / or overlay file might be named something other than the standard file names, but I’m sure from here you know where to find it.
Hope that helps