I am not too sure about the bookshelf, but as I understand it the theme is in essence a user interface - in which (if you can edit it) you can change some aspect of the organization and appearance (e.g. which directory you keep the books, fonts, etc.).
Perhaps the answer to your question is
here: if you download the source of the theme,
UnicodeMS_src.zip, there are some dictionary related lines that are those you want:
Quote:
dictionary.font.bold=#Arial Unicode MS,18,0
dictionary.font.normal=#Arial Unicode MS,18,0
dictionary.font.translation=#Arial Unicode MS,18,0
dictionary.font.extra=#Arial Unicode MS,20,0
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so you could just try
the theme itself, and see if it works.
I think in order to compile the theme after modifications you still need the theme compiling software (see
this message by Kacir)