If you'd like to remove the below rounded edges from the active tab in the menu:
 
 
 
You can simply add the below custom css to 'customize > custom css' which will change the style to the below:
 
.customerHub .chContent .chContent-Wrapper .chContent-Body .chContent-Body-Menu .chContent-Body-Menu-Block .chContent-Body-Menu-Block-Item.chContent-Body-Menu-Block-Active{
  border-bottom-right-radius: 8px!important;}
 
.customerHub .chContent .chContent-Wrapper .chContent-Body .chContent-Body-Menu .chContent-Body-Menu-Block .chContent-Body-Menu-Block-Item.chContent-Body-Menu-Block-Active{
  border-top-right-radius: 8px!important;}