Hi 360 elearning,
Lukasz' response would be the correct approach, in using the theme.less for your customisations.
However, if you do create a new file, make sure you give your styles the correct specificity, as the less files get compiled in alphabetical order. So if you create a new file advanced.lessĀ this would be compiled first and if it has the same specifity as some of the other stylesheets you won't see any changes.
I am sure you would have figured this out but just incase it saves any headaches.
Thanks,
Nils