The bibliographic management systems Mendeley, Zotero and Papers use CSL, the open source citation style standard, that allows users of different systems to exchange their citation styles.
More than 2.800 citation styles can be downloaded from the Zotero repository. Until now knowledge of XML syntax was required to customize the styles. The new WYSIWYG citation style editor makes it easier for anyone to edit or create new citation styles.

More about the CLS editor at Mendeley Blog or see
User guide for the CSL Editor
Using the Citation Style Editor with Mendeley Desktop