Adding a cpt-city color ramp
If you do not want to add a color ramp from scratch, a large collection of existing color ramps from the cpt-city archive of color gradients is available for use in QGIS. To add a cpt-city color ramp, click on the Add item button () and then choose Catalog: cpt-city. This opens the cpt-city color ramp window:

Color ramps can be selected by theme or by author by choosing the appropriate tab at the top of the window. In either case, the color ramps are presented in an expandable tree on the left, with a list of color ramps in each tree element on the right. When a color ramp is selected, the Selection and preview and Information tabs are populated.
There are two ways to add a cpt-city color ramp to the list in the Style Manager: as a cpt-city or as a standard Gradient color ramp.
To save the color ramp as a cpt-city color ramp, click on OK with a color ramp selected. This will keep the link between the added color ramp and the cpt-city color ramp list. The color ramp cannot be modified if it is added as a cpt-city color ramp.
To save the color ramp as a standard gradient color ramp, check Save as standard gradient and then click on OK. This will save the color ramp as a gradient color ramp, and the color ramp will not link back to the cpt-city color ramp collection. The color ramp can be modified later as it has been converted in to a standard gradient color ramp.