The starter kit is minimally styled so that it can be picked up and used without having to first unpick reams of css.
The starter kit is built on bootstrap, and many of the default styles are used across the site, only because we want to be on trend have we removed some of the rounded corners (they are so April)
While ideally we love to use SASS and Gulp to build and control our stylesheets, for the starter kit we wanted to keep it simple, so we are using plain old CSS, to build our styles.
We have however separated the styles out into different stylesheets so it's easier to see what's happening. we then use the Client Dependency Framework that is built into Umbraco to merge and minimize our stylesheets for super fast site serving.
this contains the imports to import all the other stylesheets in the site.
the core stylesheet sets fonts, and default colours (these can be overridden see below), it also takes the rounded corners away.
controls the little things that appear on the homepage, like the highlight box and how news and events are laid out.
contains any styles that affect the content on the site (i.e the stuff on a page). so headings, etc..
Layout-footer.css , layout-navbar, layout-breadcrumb, layout-atoz
all do what they say on the tin.
Using sections you can change how areas of the site are styles, whenever you create a section homepage (or the homepage of the site) you can specifiy a section stylesheet, that is then added to any pages in that section of the site.
This means you can do things like change the colours of things in sections or even the font, or any css styling you can think of - this Help section does this. help.css changes the colours of the help section.