At present there's no mechanism to change the theme. I could easily add this but it would result in an extra UI parameter panel to the question author's form, which is already too complicated. So I'm not at all keen unless I feel there is a strong demand, and no-one else has ever asked for this.
The language is automatically set by whatever language is being used by the question. It may even change dynamically, as in multilanguage questions. So I don't understand why yours is currently set to text. If you're using a Java question type, it should be set to Java. Can you clarify, please?