Colors, font size and copy-ability of the dictionary entries depend on what widget was used within MStarDict to display dictionary entries. Ideally, it would have been something using currently active theme for both color and font size of the text.

If I understand correctly, it's currently (see src/transwin.cpp ) :
window>alignment>pannable>vbox>several trans
trans=vbox consists of: hbox and Definition=label, where hbox consists of Expression=label and Bookname=label

So, we should be looking at style (or, even better, class - to be same as other parts of operating system) of
label = gtk_label_new("Definition");
to see any change in text color and text size. I am not sure what exactly sets the black background color, though.

[Edit]
label->set_selectable (true) around here should work. Hopefully.
[/Edit]

Thank you. Best wishes.