How to modify graphic styles in Illustrator via the appearance panel tutorial in CC 2017 2015 CS6 etc
You can modify Illustrator graphic styles via the Illustrator appearance panel (access the appearance panel via the window menu) You can find many wonderful Illustrator graphic styles including gradients on the site
You will then see all the different elements such as multiple fills and multiple strokes for the current selected graphic style - if there is no graphic style for the current selected path then the appearance will probably be a little emptier with only a single fill and stroke attached to the current path
1) Select a path
2) Apply graphic style via the graphic style panel
3) Go to the appearance panel
4) Click on the individual fill and strokes and effects entries and in the case of many of the graphic styles available on this site, you will see the gradient swatches associated with that entry and you can use the dropdown for the swatch to change the gradient or pattern or perhaps use the gradient tool to modify the position and angle and origin point etc of the gradient entry and this can be repeated along with changes to the opacity and blending mode for each entry.
A subtle change to the blending mode can radically modify the graphic style for Illustrator and of course, you can then go to the graphic styles panel and create a new style based on the settings in the appearance panel
You can also remove entries such as a fill or an effect simply by selecting that entry and clicking the rubbish bin or trash can in the bottom right corner of the appearance panel.
Removing items can also have a radical effect on modifying the graphic style and how it is applied to a path. On the left you can see a graphic style that is made from a number of different gradients as well as a few solid colors and a selection of blending modes such as lighten. Simply by changing the origin of the gradient for one of the gradients in the graphic style, the right graphic style can be seen with much the same color but with a very different internal structure (more a ring design than a crescent as in the left)
You can find other tutorials such as how to use the graphic styles as a source for symbols in Illustrator