Hello, @PureInStudio,
Simply go to the Dashboard >> XStore >> Theme Options >> Theme Custom CSS and then paste the below code in the CSS box.
.product-view-light .light-left-side{width:60% !important;}
Then save settings and check back to your site after clearing the browser cache.
The result will appear like this: https://postimg.cc/CBzkF1Sz
Regards 8Themes Team.