I had selected "Allow access to file URLs" option like in this Stack Overflow question.
After I added a custom Javascript highlightjs-line-numbers.min.js in the “Asciidoctor.js Preview” Google Chrome Extension. I found a difficult to remove the custom Javascript because I want to display line number to the Asciidoctor Source Code Block. The problem appear when I add custom JavaScript via "Options" link of the extension.
How to remove the previously added custom javascript in Ascidoctor.js Preview options, for example highlightjs-line-numbers.min.js ? But I can remove the previously added "a custom cascading stylesheet (css)". In the picture, you can see Remove button at the right side of Theme/Stylesheet combo box. 
Am I missing something?

highlightjs-line-numbers.min.jsdidn't work without showing us the contents - how are we supposed to know? And the 2nd because you are asking 2 separated clearly different questions. Please dit the question to make it more focus and detailed.