Editing Reference Styles in ArcMap?

Here is some "hack" method. But check if it complies with licensing policy.

We can find ArcMap referenced styles in file c:\Program Files\ArcGIS\Desktop10.0\Styles\ESRI.style.

It is standard ArcGIS style library file.

Soon, I have realized that it is actually MS Access file! Just change it's extension to .mdb and try to open it in Access.

You will find all styles and I believe you can make necessary modifications to it. Then again just change extension to .style

Go to Style Manager under Customize. In your own custom style-list: enter the folders that are not greyed-out. Here you will find your custom created styles. Edit and delete at will.