To change of dictionary articles presentation you can create the file with styles which will be used while articles output. Such file must be named "article-style.css" and placed in GoldenDict configuration folder beside the "config" file. The "article-style.css" is just style sheet file wrote in accordance with standard html style sheet rules. To see a class names used in articles you can call inspector via article context menu.
Also you can tune GoldenDict interface. To make it you should create the "qt-style.css" style sheet file in GoldenDict configuration folder beside the "config" file. The rules for Qt style sheets you can find here.
The examples of "article-style.css" and "qt-style.css" files you can find in GoldenDict source code.