Using the style editor to alter a local style

If I have a local style in an XHTML document, how can I edit it with the
style editor? Selecting the style definition in the source code, or
selecting the part of the document to which the style is applied does not
seem to give me any easy option to alter the style itself other than to
write it by hand in the source code.

In particular, selecting Style/CSS Style... only gives me a blank sheet in
which I can create an entirely new style; I would rather sue the same tools
to edit what is already present.

What am I doing wrong? Or isn't it possible to do this?

JDL