I think you have just discovered one of the limitations of the 505. I tried full justification rather than right justified, both in the css and via Sigil. As you say, looks fine in Sigil. Not on the 505.
There will be those that shudder at the idea, but nowadays I format everything in Open Office and export as pdf. You can have any font size or shape you like then, and, if you really want, as many fonts as you like.