Implement custom theming support

[Still34]

Since the tooling currently supports the --theme parameter, it would be great if the user could specify a template or theme folder during page build. At the moment, it is not possible to allow for user-created themes to be used with papoGen without appending their themes within the lib folder, which is not ideal for CI-generated documentations.

docfx has a great implementation for user-defined CSS/partial templates. This design system allows for the user to supply their own CSS, overwrite the default theme, and even merge templates together (e.g. with `--theme "default, custom-theme", docfx would automatically merge and overwrite files depending on which theme was referenced here). Here is the docfx template design specification. If possible, behavior similar to such would be great - or even something as simple as just letting the user provide their own stylesheets.