With LESS I divided my styles in different stylesheets which were imported into one main LESS file and it was divided something like this:
Because I worked with more Front End Developers, we had this structure for each project, which made it a whole lot easier to fix and add new things.
In my new job I don' t have a preprocessor and I'm the only Front End Developer, but I keep the same structure as above, but then in a single CSS file. At the top there's a Table of Contents (as comments obviously) and I can use that to search for a specific part or component.
But that's just how I work (and worked)
There are a lot of frameworks out there, but don't follow them blindly. And the naming conventions too. BEM, SMACSS, ITCSS, ACSS, OOCSS... it's all good if you (and your team!) stick to one of these though.
Just don't linger on it too much. In the end it only matters if you write correct CSS which works in most of the browsers ;) . The structure and naming is secondary I think. In the 15 years I've been building websites, no CSS file has ever gotten me in a situation that I couldn't edit it or find a particular selector.
CTRL-F is your friend ;)