Thanks for considering contributing to the Harp website.
This version of harpjs.com uses Bootstrap. Bootstrap is in _bootstrap and is not modified.
To override the default behaviour of Bootstrap, make a copy of the component in _override and import it. Make the changes on top of Bootstrap, removing the lines that will not change from the original.
For example, the main banner on harpjs.com uses a .jumbotron class from Bootstrap, which is located in _bootstrap/jumbotron.less. This is augmented with additional styles, like the additional .jumbotron-knockout class, in _overrides/jumbotron.less
For quick and less modular changes, work with in a .section in application.less, using non-Bootstrap classes.