You can read in static css with <mx:Style or dynamic css
with StyleManager.loadStyleDeclarations(). That's good but the
problem is that the latter reads in a compiled .swf file instead of
a textual .css file. The person who is modifying css may be a
designer, not a programmer. It is a pain to say that the designer
has to install the Flex SDK and know how to run the compiler. Is
there some way to load a .css file at runtime?
I sent the question to Adobe tech support, and got an answer
saying dynamically loading a CSS file is a feature they have talked
about, but for now it hasn't been added. It sounds like for now if
you want to work with a non-programmer who will edith the CSS, you
may have to set up a script that runs the command-line compiler,
and then puts the compiled result in the right place. This would
include an install of the compiler on the designer's machine. This
is a pain but not completely impossible.