Set font weight for a custom classification

I posted the same question at StackOverflow, but thought I will ask here too. I cannot find a way to set non Bold font weight for custom classification in VSX. I tried to use TextFormattingRunProperties.SetTypeFace and a couple of system fonts with unregular weight in the code below : 

...
var formatting = classificationFormatMap.GetExplicitTextProperties(classificationType);
// newTypeFace is a one of System fonts with non Normal weight, which was received before    
formatting = formatting.SetTypeface(newTypeFace); 
classificationFormatMap.SetExplicitTextProperties(classificationType, formatting);

but weight didn't affect in the visual studio text editor window. However, the font families and styles were applied very well.

Has anyone idea why SetTypeFace with weight doesn't work and how to sent font weight? Did I miss something?