Wedding Theme

Navy blue would be fine! Just pair it with lighter colors. You could do navy for the sky, a cream color for the moon and gold for the stars. You can have this as subtle or as bright as you want. The gold could just be little accents everywhere like glittering gold votives for candles. Your flowers could be a neutral cream, maybe a fun shape like orchids, something “star” shaped. You could have a beautiful moon backdrop for your cake table. You could hang lights that look like stars. You could include astrology for a fun thing for your guests. It’s a great theme and you could do a lot with it!

Navy blue would look great with soft colors such as lavender to create a moonlight or galaxy look.

I agree, navy will look lovely in July. Love the idea of gold accents. White & greenery would be great, or perhaps blush with the navy.

The moon cake topper will be great! Will you rent or make a giant 1/2 moon? Could be used as a photo backdrop somewhere at your reception.

I agree that navy would not be too dark if you pair it with lighter colors. You can make the theme subtle or more obvious as you choose. Here are some ideas.
Blue and copper

Celestial wedding ideas

Moon theme wedding

Navy is always appropriate. Our colors were white, navy and gold and we got married in July!!

A Starry Night theme wedding popped into my head while reading your post! Navy would be totally fine in July