Following the previous answers, in most cases I would stick to the WordPress default styles and patterns — for design and also markup, functions, APIs and cod, not only the CSS –, specially if it’s a plugin to the general public and to be hosted in the official repository.
- As Milan pointed out “There are many cases where WordPress admin styling is not enough.”
- As said by Mark, “users hate to learn the new “snow flake” UI, especially when they have 20 plugin each with its own “snow flake” design.”
- Your plugins will be more “future-proof” like. For example, if WordPress changes it’s default CSS and styles in the future, your plugin will follow automatically in most cases.
That said, there will be times you will need to tweak, or event change completely, something. My advice is to build upon what WP does and change just what really needed or justified.
My company manages 3 WordPress multisite networks: A for our own clients, B for a specific niche managed sites/CMS/Digital Hub SAAS and C for other boarder niche managed sites SAAS.
- Everything we make in A follows WordPress styles by the book.
- On B we dropped most of WordPress styles/patterns for an APP/Material Design like approach. We wanted the user to feel user an app and not WordPress because it makes sense for the service. We changed a lot of design/presentation stuff with CSS but keeping as much of WP HTML markup andPHP/Javascript code patterns as we could.
- On C we dropped WordPress styles/patterns in a few places.
And for plugins we release “in the wild”, most of time we follow WP style/patterns.