in geoserver, Styled Layer Descriptor (SLD) which is a XML-based markup language uses for styling any geospatial data. up to the present i have never come across with Styler for Geoserver. but you can use some tools with converting and adding some codes to core in it for your needs.
1.GeoServer Styler plugin -- example
Styler is built on open standards, using the Styled Layer Descriptor
(SLD) XML language at the center. It is rendered with the Web Map
Service (WMS) standard, which has extensions to render remote SLDs,
and uses REST calls to persist styles to servers. It is built with
GeoServer as an initial target, but the emphasis on open standards and
flexible design should make it adaptable to any WMS server.
guide for some tutorial here.
Download the REST plugin for your version of GeoServer (1.7.1 or newer)
from the download page.
Unzip the archive into the WEB-INF/lib directory of the GeoServer installation.
Download the GeoExt Styler extension from the Download page.
Unzip the archive into the www directory of the GeoServer data directory.
2.AtlasStyler SLD editor
AtlasStyler is a user-friendly application to style geodata. The
resulting styles can be saved to XML files that follow the OGC SLD/SE
The AtlasStyler is a "stand-alone" application, that can be used
independently for any vector data. ESRI Shapefiles, OGC WFS and
PostGIS datasources are supported. Automatic upload of the SLD styles
to Geoserver will be possible soon.
3.GeoServer Style Editor
Standalone - editor is not tied to a particular program (uDig, MapBuilder, GeoServer)
Visual - users can preview the results of changes as they are made.
Intuitive - easy to learn but not cumbersome or limiting for the advanced user.
i hope it helps you...