Universal themes are those that transcend time and place and tap into something timeless and essential to the human experience. It is the universal elements of Shakespeare's works that have made his plays and poems the classics that they still are to this day.
Sonnet 116, "Let me not to the marriage of true minds," is one of Shakespeare's most famous poems. It is frequently read at weddings and printed on valentines. Clearly, Shakespeare tapped into something universal with these lines that make the sonnet still relevant today. The primary theme of this poem is that true love conquers all. In it, Shakespeare argues that real love will always prevail even in the face of the harshest challenges. Clearly, this is a reassuring and uplifting message that many still need to hear today. This message is not specific to any single culture or limited in scope to only speak to an audience of Elizabethan-era readers, but instead speaks directly to the human experience.
Other universal themes that Shakespeare explores across many of his sonnets include aging and leaving a legacy, art and writing, beauty, jealousy, infidelity, and morality.