Glossary has restricted capabilities for text formatting. This is by design, so that you are not distracted when writing and can focus on the content.
This is especially helpful when copying and pasting information from other sources, because you do not have to fiddle with fonts and font sizes: these are automatically adapted to those used by Glossary which allows only three given font sizes (small/normal/large).
Only the following features can be changed:
The font family (e.g. Times or Helvetica) to be used can be changed in the preferences. Existing documents can be converted to use the new font settings. For printing other font families are used than on screen. These can be set in the preferences as well.