You need to choose the element, by either id (#), class (.), or semantic (header, footer, nav etc.).
These can be combined with selectors to be even more specific, for example p{} will style all paragraphs, however body p{} will only style paragraph elements within the body. CSS has entire concepts devoted to this, and it's worth reading up on how both the cascade and specificity works (rules like !important will help you a lot in the beginning, but try not to become reliant on it!); as well as additional selectors like siblings, adjacents, and wildcards, as well as pseudo classes.
The best way to identify elements in C3 is open dev tools in Chrome and use the selector tool (Ctrl+Shift+C) to highlight elements, for example the new file button is .spOpenFileButton, so to give that a red background you would write:
.spOpenFileButton{
background:red;
}