If you do the installation through Yeoman you will have a css folder structure like this:
_config/ devices/ helpers/ classes/ functions/ mixins/ placeholders/ modifiers/ objects/ ui/
These folders are just for your project, all the FrontendCore folders and files are inside the components folder and you should never modify anything inside there. There are several ways to customize the objects or even disable them to get out of your final css file. Check the object documentation to know how. Let's take a look to all the folders and what should them contain.
This folder must contain just config files: variables & maps. In the final compilation this folder should not generate any css code, is just to define your global variables or FrontendCore variables/maps to customize the objects.
All the variables inside FrontendCore are defined with !default, so if you define them before call FrontendCore you will be able to override anything.
Here you will find the media queries. By default there are three breakpoints (mobile, tablet & desktop), but you can modify them or create new ones as you need. Just remember to add them to the _index.scss file on this folder.
This files should not contain any code apart of the media-query definition. Check the Media-queries documentation to learn more.
Here you should add your global helpers. In the final compilation this folder should not generate any css code.
There are 3 types of helpers:
By global we mean about helpers that can be used in several objects or situations. If you for example have a placeholder used to create different buttons, probably is best idea to have this placeholder inside the button object folder.
Here you should add your customs modifiers. A modifier is a small class that just do one thing but with the !important statement. By default FrontendCore provides you a lot of modifiers but if you want to add more this is the place.
Here you should add all your objects. You should create a folder for each object and include them in the __index.scss file you will find in this folder.
All the FrontendCore JS components that need CSS are calling asynchronous the ui.css. If you want to do the same with your JS components just add in this folder a new folder for your component scss and add it to the __index.scss. You also will need to setup your JS module to call the css on loading.