the Vizlib objects are pretty extensive with customization. one thing you can do is go to the vizlib support pages for the custom report (just google vizlib custom report documentation)
futhermore, in the screenshot you are currently selecting the container object. if you want to change anything within the custom report, you need to select the custom report.
best thing to do (in my opinion) remove the vizlib container object so you can work in the custom report
after that you can go to the appearance and change the settings to your liking.