How to dress up glass cabinet doors?
Stencil with crystal paint
If you don't want to see inside. The mirror effect spray paint is a good solution.
They gave window decals that are really pretty. You can find them in so many designs and it's really easy to use. you can also use adhesive vinyl to decorate the glass. There is glass paint at most craft stores also.
Style the cupboards behind the glass cabinet doors in an attractive way, or you can add frosted contact paper to hide what is behind them, or hang lace curtains behind the glass doors.
Paint "muntins" onto the glass, or stencil art deco or similar motifs onto the glass, using a color/colors of your choice.