Wouldn't it be nice if you could make an attribute in your style sheet, like a road color or a font size, into a variable which you could change from your application? There's already a way to do this using style overrides, but here's a simpler and more efficient way.
Invent a name for your variable, like 'roadcolor' and use XML entity syntax, which means you write it as &roadcolor; - put an & in front and a ; after it. Then you can use it anywhere in your style sheet, like this:
<line width='12m,0.5pt' fill='&roadcolor;' border='dimgrey' borderWidth='8%,0.7,2'/>
To make it work, you have to give it a value. Call CMap::SetStyleSheetVariable:
You can change the value at any time by calling CMap::SetStyleSheetVariable, then, after you have made any other changes to style sheet variables, call CMap::ClearStyleSheet to tell CartoType to reload the style sheet before the next drawing operation.