In this article we’ll see how to insert your own CSS in a Sharepoint 2013 page.
Open the sharepoint page for editing that you want to paste your CSS to
Select Embed Code option from Insert tab in the ribbon
Paste your CSS into the text area in the dialog box and press Insert.
To assign CSS classes to the page elements, use Edit Source option in FORMAT TEXT tab in the ribbon. Assign the CSS classes to the required html elements.
Save the page and exit the edit mode. Your page should now display the content with CSS styles applied.