A: YES! the themes only depends on CSS. You can develope your own ones via pure CSS, LESS or SCSS. But in case you want to contribute to the project, using of the build-system and default theme structure is mandatory
A: NO! Compared to EnlighterJS v2 it is not longer possible to add custom language files to your page by just including the javascript files. This is caused by the new systems architecture (monolithic library) of EnlighterJS v3 - everything (classes, namespaces, languages) is isolated from the page and not exposed as global variable.