1920s style

I have seen Great Gatsby themed weddings when looking at themes. Try searching that, there is a lot of art decco, black and golds. Flapper styled dresses with lots of beading. Looks really cool.

I think a lot of people are looking into Gatsby themed parties with 2020 coming soon, it should be easy to find decorations and ideas. Here are a few dress inspo pics. As for asking guests to dress to match, I would suggest wording it along the lines of "Gatsby/20s attire encouraged". You can also set the tone with your invitations with an art deco theme.
Accessories make all the difference

Themed dresses

Hey there, Audriana!

I love this theme. Very glamorous

Here are some inspo pics I found on Pinterest!

Your girls could wear pearl headbands, or fabric headbands with feathers around their heads like in the photos above! The addition of the pearls really create that glam 1920's feel.

With the dress code, I agree with the PP. You can totally set the theme with the design of your save the dates and invitation. You could add a line advising of the theme and if they would like to dress up - "1920s attire encouraged"

Do you want the dress code to be optional?

Enjoy planning. How could you not have fun with such a great theme.