In a standard system the stage chevron styles are defined in the py-portal-stages css which is attached to the skin rule as an additional stylesheet (Skin Rule / Included Styles tab / Additional Style Sheets) as shown below.  

The $mx-link-color syntax is referring to a Mixin Variable.  This one is referring to the mixin ("mx"), called "link", and the style "color":

So you could override this mixin to change the colour.  However, be aware that this will change the colour of anything that uses this mixing so you will find that anything that uses that same colour will also change.

Alternatively you could override the chevron style to use a different new mixin, or specifiy the style directly rather than using a mixin variable (not recommended).

There is some more information about this in the article Mixins in an override CSS