Based on your reply you are editing the theme code (tag or markup). If you are editing the theme code and writing your own markup then you have to write the CSS based on your code (tag or markup). Because there has nothing that will work automatically with your edited code (tag or markup).
I didn’t change a single line of code in a theme, and will probably create a child theme if I need to change anything.
No plugins outside of required once are installed.
I’m just editing example data.
check it here www1.vatacom.com , scroll down to Zultys phone system