Hi,

1/ Unfortunately, currently this theme doesn't support display only main category. Because almost customers always want to display full the categories of posts.

2/ Please try add more this code to Customize > Custom CSS to do that:

\
.cat > a.penci-cat-name{ display: inline-block; background: #f43192; color: #fff !important; padding: 2px 5px !important; font-size: 11px; margin: 0 7px 0 0 !important; }\
.cat > a.penci-cat-name:after{ content: none; display: none; }

Change color code f43192 on the above code to any color code you want. You can get a color code here

Best Regards,

PenciDesign