What does the Bible say?
The Bible never mentions henna by name. The closest passage is Leviticus 19:28, where God gave ancient Israel a command not to make bodily marks in connection with mourning for the dead. That instruction sat among laws that set Israel apart from the religious customs of surrounding nations.
Christians differ over how directly that command applies to modern tattoos, and henna is usually temporary rather than permanent. Still, the heart of the passage is worth hearing. God did not want his people joining practices that expressed allegiance to other gods or distorted their worship of him (Leviticus 19:28).
Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the Lord.
Your body and your purpose
Paul tells Christians in Corinth that their bodies belong to the Lord and are places where God's Spirit dwells. His immediate context is sexual sin, not body art, so this passage doesn't give a quick rule about henna. But it does give a larger question: does this choice fit with treating my body as belonging to Christ (1 Corinthians 6:19-20)?
A henna pattern for a wedding, celebration, or simple enjoyment may be very different from a symbol chosen to identify with a belief or spiritual practice opposed to Jesus. Ask what the design means, why you want it, and what you hope it says. Then aim to do everything in a way that honours God (1 Corinthians 10:31).
What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.
Freedom needs love
Not every permitted thing is helpful. Paul wrote those words to a church that needed to learn that personal freedom should build up other people, not merely satisfy personal preference (1 Corinthians 10:23-24). That gives a helpful guide where Scripture does not give a direct command.
For example, perhaps henna is normal and innocent in your family or culture. Or perhaps a particular pattern is strongly associated with a religious ritual you don't wish to endorse. Context matters. A Christian can enjoy good things with gratitude, while choosing not to use freedom in a way that confuses or wounds another person's faith (1 Corinthians 8:9).
Make a clear-hearted choice
You don't need to live anxiously over every appearance choice. Yet don't brush aside an uneasy conscience either. Paul says that actions not done from faith are sin, which means you should not pressure yourself into doing something you cannot do before God with honesty and thanks (Romans 14:23).
Pray about it. Consider the design and setting. If you are uncertain, talk with a mature Christian who knows you well. You may choose henna with a peaceful conscience, or you may decide it is not for you. Either way, let love for Christ, rather than fear or pressure, shape your decision (Colossians 3:17).
And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.