1.Open SharePoint Designer and connect to the sitecollection where you wish to add the custom style

2.After connecting to your sitecollection with SharePoint designer, in the left navigation section click on All Files and after that click on Style Library on the right.

3.Right click somewhere in the white area on the right, click on New and then on CSS.

4.Give the CSS file any filename you would like, I named mine custom.css.

5.Right click on the CSS file you have just added and click Open to open the file in edit mode.

6.Now add a style section in this CSS file for each style you wish to add to the ribbon. There are two sections in the styles section in the ribbon you can add your custom style to: Page Elements and Text Styles.