The style and template files are regenerated every time NodeBB reloads. To change them permanently, you need to either edit the style.less of the theme you are using, or enter your custom css under the Appearance=>Custom HTML and CSS tab in the admin panel.
I recommend using the Custom CSS option for small changes like this, it's easier and doesn't require a reload.
Sadly, I wasn't able to find that option ? Where it should be?
Is this the right file to edit? (I'm using the "persona" theme)