The base scss file is placed in the sass folder of the FrameworkCoreBundle. The layout is based on Bootstrap, so don't create new components when there's already a Bootstrap component available. Also try to customize as much of the components as possible through the bootstrap-variables file. This makes the code easier to maintain.
If it's not possible to customize a Bootstrap component through the bootstrap variables, you can create a new file in the components folder where you override the necessary properties. This has already been done for many components to create our own layout. If you want to customize the layout you can edit the existing code in the sass folder, or you can create new components or layout styles by adding and importing new files.
All scss files from all bundles placed in the src directory will be put together, which means you can import styles from other bundles from within FrameworkCoreBundle. Bower components are also added to the import path, so you can easily import sass files from plugins when available. Try to use Bower as much as possible for frontend plugins to make upgrading and maintaining easier.