This most likely won't be added. It would require either taking the
site's CSS and merging it into the resulting e-book file (which might result in strange layout issues etc.), or creating special cases in the adapter to transform the span-tags to either contain their CSS directly in a style-attribute or turn them into font-tags with a matching color-attribute (even if inaccurate) -- which both is hard to automate without a DOM-aware HTML parser (such as a real browsers) or a proper CSS parsing library as far as I know. Then the question also comes up where to stop: Should we also add custom fonts, and other strange CSS definitions for other sites etc.
But maybe Jimm has a completely different opinion, so wait for his response too.