I'm setting up a stylesheet and and finding the process of defining related styles tedious and error prone. In particular, I have several related styles that need to have synchronized settings (e.g., such as "DisplayFormula" and "DisplayFormulaNumbered"). Is there a way to base one style on another, or perhaps simply to copy or duplicate an existing style as the starting point for another?
This required editing the cell expression (Ctrl+Shift+E, or Cmd+Shift+E on Mac), but it's exactly what you're looking for:
To do this, your stylesheet has to either contain the parent style or inherit from another stylesheet which contains the parent style.
You can see many examples of this in the built-in Mathematica stylesheets, such as the "PrintTemporary" style (based on "Print"), "Graphics3D" (based on "Graphics"), and many of the hyperlink-type styles which inherit from "Link".
Note: stylesheet inheritance is available in Mathematica v6 and later. My examples above were referenced from v8 stylesheets, but may not apply to earlier versions.