By stroke of the cubic bezier curve I mean rendering a curve 'A' with a specific line width 'w'.
How can I derive other cubic bezier curves that describe the outline of the stroke of bezier 'A' ?
Answer
Ohhh. You want to get the offset-curves of an bezier curve.
Bad news. this is hard because these curves can't be simply derived numerical. They contain all kinds of intersections, loops and other nasty stuff.
There are some approximations though. The best approach I've read so far is from a paper by Thomas F. Hain (Fast, Precise Flattening of Cubic Bézier Path and Offset Curves).
He does flattening, so his paper is mostly about decomposes the offset curves into line-segments and circular arc-segments, but you can merge them back to beziers later.
For better understanding you may want to read his other bezier related papers as well.