Let me say happy 18th birthday for you who have entered maturity period. You may need 18th birthday party ideas. Below we have a few awesome 18th birthday theme suggestions that you might use.
Hawaiian party theme
A Hawaiian party theme can be a fun and exciting theme for an 18th birthday party. Every single guest requires being dressed ala Hawaii, like a tropical t-shirt, khaki shorts and maybe sunglasses 
Make sure the party area decorated with colorful theme. Would be perfect if the parties carried out at the beach. So awesome
Disco theme
Young people are usually very pleased with the party who performed late into the night. At your happy 18th birthday, you can create your own party with disco theme. Setting the party area with the blinking lights combined with a dark atmosphere. Invite all your friends and let’s rock the night!!
Cowboy theme
One of the creative 18th birthday party ideas. Can be done in a park and serve barbeque. Make sure that the guests who came wearing a cowboy hat 
Prepare a lot off grill because you will need it and enjoy your 18th birthday party.
I hope some 18th birthday party ideas above can help you who are doubtful to determine the theme of your birthday party. Here is my previous post for who looking for 18th birthday gift ideas. Once again, happy 18th birthday