Whatever you might think of Drupal, its version 8 will likely change your mind.
Here’s the Drupal 8 standard CSS Architecture. I’ve seen quite a few style guides and architectural documents; this one is well done. If you value collaboration and ease of development, you’ll want to use a similar document. Don’t write code for yourself: comment liberally with lots of white space. You can always remove that stuff later.
The entire Develop for Drupal documentation is a nice read.