NikolayIvanov Flarum extensions work in the backend via a mechanism called "extenders", which are kind of a declarative way of establishing what the extension does. In practice, this looks like an array of instances of Flarum/Extend/SomeExtenderName classes, with various methods called. (although there's nothing magic about that namespace; extensions can add their own extenders too).
Individual Flarum installations also come with an extend.php file, where you can add extenders outside of an extension. The rationale for this is supporting lightweight per-site customizations without needing to publish and maintain an extension as a package.
For less, all you need to do is add the following to your extend.php array:
(new \Flarum\Extend\Frontend('forum'))
->css(PATH_TO_YOUR_CUSTOM_LESS_FILE),
That being said, I am curious: why not just use the appearance section of the admin dashboard?
